On Mon, Jan 18, 2021 at 03:32:57PM -0500, Al Cooper wrote:

On Mon, Jan 18, 2021 at 12:45 PM Greg Kroah-Hartman
[off-list ref] wrote:

quoted

On Fri, Jan 15, 2021 at 04:15:43PM -0500, Al Cooper wrote:

quoted

Add a UART driver for the new Broadcom 8250 based STB UART. The new
UART is backward compatible with the standard 8250, but has some
additional features. The new features include a high accuracy baud
rate clock system and DMA support.

The driver will use the new optional BAUD MUX clock to select the best
one of the four master clocks (81MHz, 108MHz, 64MHz and 48MHz) to feed
the baud rate selection logic for any requested baud rate.  This allows
for more accurate BAUD rates when high speed baud rates are selected.

The driver will use the new UART DMA hardware if the UART DMA registers
are specified in Device Tree "reg" property. The DMA functionality can
be disabled on kernel boot with the argument:
"8250_bcm7271.disable_dma=Y".

Shouldn't that be on a per-device basis, and not a per-driver basis?

There is only one instance of the UART DMA hardware and it gets muxed
to just one of the possible UARTS.

But the driver doesn't know/care about that, it binds to any device that
matches it.  per-module/driver flags are not a good idea.

quoted

And why would you want to disable this, if you have support for this in
the DT?  Why not just rely on the DT setting?

The DMA feature is used when the UART is connected to a Bluetooth
controller and the BAUD rate is typically 2-3Mbs. The ability to
easily disable DMA is very useful when debugging BT communication
problems in the field. DT settings could also be used to disable DMA,
but knowing the correct modifications to the "reg" and "reg-names"
properties is a lot more complicated.

So this is a debug-only option?  If so, why not just make it a debugfs
file then?  No need to clutter up a module parameter for this mess.

thanks,

greg k-h