Re: [PATCH 09/10] ARM: bcm2835: Add the mailbox property channel driver.

Stephen Warren [off-list ref] writes:

On 03/02/2015 01:54 PM, Eric Anholt wrote:

quoted

Many of the operations with the firmware are done through this mailbox
channel pair with a specific packet format.  Notably, it's used for
clock control, which is apparently not actually totally possible to do
from the ARM side (some regs aren't addressable).  I need clock
control for the VC4 DRM driver, to turn on the 3D engine.

quoted

diff --git a/drivers/mailbox/bcm2835-mailbox-property.c b/drivers/mailbox/bcm2835-mailbox-property.c
+int bcm_mbox_property(void *data, size_t tag_size)

quoted

+	buf = dma_alloc_coherent(NULL, PAGE_ALIGN(size), &bus_addr, GFP_ATOMIC);
+	if (!buf)
+		return -ENOMEM;

Can't the driver (this one or the client) maintain some persistent
buffer rather than allocating/freeing a new one each time. It seems like
the alloc/free might introduce quite some overhead?

The size of the buffer is arbitrary (up to 1MB), the frequency of
requessts is low, and the hardware's pretty starved for RAM.  However,
we're probably only ever going to see single page allocations, so the
RAM cost is probably low and the allocation time is probably also
correspondingly low (not like when I was trying to do 256k allocations
in the vc4 driver.  ouch).

quoted

+	writel(size, buf);
+	writel(bcm_mbox_status_request, buf + 4);
+	memcpy_toio(buf + 8, data, tag_size);
+	writel(bcm_mbox_property_end, buf + size - 4);

Since this is just a regular chunk of RAM, can't the code just use
regular memory writes and memcpy()?

will wmb() guarantee that the compiler won't optimize out my write to
buf[0] and corresponding read from buf[0]?  I was originally treating it
as RAM and using volatile, but then reread the old "don't use volatile"
doc.