Hi Tomi,

On Friday 17 August 2012 13:51:49 Tomi Valkeinen wrote:

On Fri, 2012-08-17 at 12:02 +0200, Laurent Pinchart wrote:

quoted

On Friday 17 August 2012 12:03:02 Tomi Valkeinen wrote:

quoted

On Fri, 2012-08-17 at 02:49 +0200, Laurent Pinchart wrote:

quoted

+/*
----------------------------------------------------------------------
---
---- + * Bus operations
+ */
+
+void panel_dbi_write_command(struct panel_dbi_device *dev, unsigned
long
cmd) +{
+	dev->bus->ops->write_command(dev->bus, cmd);
+}
+EXPORT_SYMBOL_GPL(panel_dbi_write_command);
+
+void panel_dbi_write_data(struct panel_dbi_device *dev, unsigned long
data) +{
+	dev->bus->ops->write_data(dev->bus, data);
+}
+EXPORT_SYMBOL_GPL(panel_dbi_write_data);
+
+unsigned long panel_dbi_read_data(struct panel_dbi_device *dev)
+{
+	return dev->bus->ops->read_data(dev->bus);
+}
+EXPORT_SYMBOL_GPL(panel_dbi_read_data);

I'm not that familiar with how to implement bus drivers, can you
describe in pseudo code how the SoC's DBI driver would register these?

Sure.

The DBI bus driver first needs to create a panel_dbi_bus_ops instance:

static const struct panel_dbi_bus_ops sh_mobile_lcdc_dbi_bus_ops = {

        .write_command = lcdc_dbi_write_command,
        .write_data = lcdc_dbi_write_data,
        .read_data = lcdc_dbi_read_data,

};

Thanks for the example, I think it cleared up things a bit.

As I mentioned earlier, I really think "panel" is not right here. While
the whole framework may be called panel framework, the bus drivers are
not panels, and we should support external chips also, which are not
panels either.

I agree. I've renamed panel_dbi_* to mipi_dbi_*.

quoted

quoted

I think write/read data functions are a bit limited. Shouldn't they be
something like write_data(const u8 *buf, int size) and read_data(u8
*buf, int len)?

Good point. My hardware doesn't support multi-byte read/write operations
directly so I haven't thought about adding those.

OMAP HW doesn't support it either. Well, not quite true, as OMAP's
system DMA could be used to write a buffer to the DBI output. But that's
really the same as doing the write with a a loop with CPU.

But first, the data type should be byte, not unsigned long. How would
you write 8 bits or 16 bits with your API?

u8 and u16 both fit in an unsigned long :-) Please see below.

And second, if the function takes just u8, you'd need lots of calls to do
simple writes.

I agree, an array write function is a good idea.

quoted

Can your hardware group command + data writes in a single operation ? If
so we should expose that at the API level as well.

No it can't. But with DCS that is a common operation, so we could have
some helpers to send command + data with one call.

Agreed.

Then again, I'd hope to have DCS somehow as a separate library, which would
then use DBI/DSI/whatnot to actually send the data.

I'm not quite sure how easy that is because of the differences between
the busses.

quoted

Is DBI limited to 8-bit data transfers for commands ? Pixels can be
transferred 16-bit at a time, commands might as well. While DCS only
specifies 8-bit command/data, DBI panels that are not DCS compliant can
use 16-bit command/data (the R61505 panel, albeit a SYS-80 panel, does
so).

I have to say I don't remember much about DBI =). Looking at OMAP's
driver, which was made for omap2 and hasn't been much updated since, I
see that there are 4 modes, 8/9/12/16 bits. I think that defines how
many of the parallel data lines are used.

SYS-80 also has an 18-bits mode, where bits 0 and 9 are always ignored when 
transferring instructions and data other than pixels (for pixels the 18-bits 
bus width can be used to transfer RGB666 in a single clock cycle).

See page 87 of 
http://read.pudn.com/downloads91/sourcecode/others/348230/e61505_103a.pdf.

However, I don't think that matters for the panel driver when it wants
to send data. The panel driver should just call dbi_write(buf, buf_len),
and the dbi driver would send the data in the buffer according to the
bus width.

According to the DCS specification, commands and parameters are transferred 
using 8-bit data. Non-DCS panels can however use wider commands and parameters 
(all commands and parameters are 16-bits wide for the R61505 for instance).

We can add an API to switch the DBI bus width on the fly. For Renesas hardware 
this would "just" require shifting bits around to output the 8-bit or 16-bit 
commands on the right data lines (the R61505 uses D17-D9 in 8-bit mode, while 
the DCS specification mentions D7-D0) based on how the panel is connected and 
on which lines the panel expects data.

As commands can be expressed on either 8 or 16 bits I would use a 16 type for 
them.

For parameters, we can either express everything as u8 * in the DBI bus 
operations, or use a union similar to i2c_smbus_data

union i2c_smbus_data {
        __u8 byte;
        __u16 word;
        __u8 block[I2C_SMBUS_BLOCK_MAX + 2]; /* block[0] is used for length */
                               /* and one more for user-space compatibility */
};

Helper functions would be available to perform 8-bit, 16-bit or n*8 bits 
transfers.

Would that work for your use cases ?

Also note that some chips need to change the bus width on the fly. The
chip used on N800 wants configuration to be done with 8-bits, and pixel
transfers with 16-bits. Who knows why...

On which data lines is configuration performed ? D7-D0 ?

So I think this, and generally most of the configuration, should be
somewhat dynamic, so that the panel driver can change them when it
needs.

quoted

quoted

Something that's totally missing is configuring the DBI bus. There are a
bunch of timing related values that need to be configured. See
include/video/omapdss.h struct rfbi_timings. While the struct is OMAP
specific, if I recall right most of the values match to the MIPI DBI
spec.

I've left that out currently, and thought about passing that information
as platform data to the DBI bus driver. That's the easiest solution, but I
agree that it's a hack. Panel should expose their timing requirements to
the DBI host. API wise that wouldn't be difficult (we only need to add
timing information to the panel platform data and add a function to the
DBI API to retrieve it), one of challenges might be to express it in a
way that's both universal enough and easy to use for DBI bus drivers.

As I pointed above, I think the panel driver shouldn't expose it, but
the panel driver should somehow set it. Or at least allowed to change it
in some manner. This is actually again, the same problem as with enable
and transfer: who controls what's going on.

How I think it should work is something like:

mipi_dbi_set_timings(dbi_dev, mytimings);
mipi_dbi_set_bus_width(dbi_dev, 8);
mipi_dbi_write(dbi_dev, ...);
mipi_dbi_set_bus_width(dbi_dev, 16);
start_frame_transfer(dbi_dev, ...);

I'll first implement bus width setting.

quoted

quoted

And this makes me wonder, you use DBI bus for SYS-80 panel. The busses
may look similar in operation, but are they really so similar when you
take into account the timings (and perhaps something else, it's been
years since I read the MIPI DBI spec)?

I'll have to check all the details. SYS-80 is similar to DBI-B, but
supports a wider bus width of 18 bits. I think the interfaces are similar
enough to use a single bus implementation, possibly with quirks and/or
options (see SCCB support in I2C for instance, with flags to ignore acks,
force a stop bit generation, ...). We would duplicate lots of code if we
had two different implementations, and would prevent a DBI panel to be
attached to a SYS-80 host and vice-versa (the format is known to work).

Ah ok, if a DBI panel can be connected to SYS-80 output and vice versa,
then I agree they are similar enough.

Not all combination will work (a SYS panel that requires 16-bit transfers 
won't work with a DBI host that only supports 8-bit), but some do.
 

quoted

We might just need to provide fake timings. Video mode timings are at the
core of display support in all drivers so we can't just get rid of them.
The h/v front/back porch and sync won't be used by display drivers for
DBI/DSI panels anyway.

Right. But we should probably think if we can, at the panel level, easily
separate conventional panels and smart panels. Then this framework wouldn't
need to fake the timings, and it'd be up to the higher level to decide if
and how to fake them. Then again, this is no biggie. Just thought that at
the lowest level it'd be nice to be "correct" and leave faking to upper
layers =).

But we would then have two different APIs at the lower level depending on the 
panel type. I'm not sure that's a good thing.

-- 
Regards,

Laurent Pinchart