Re: [patch] HWUART instead of BTUART on the h5550 (workaround for Bluetooth UART overruns)

On Thu, 10 Mar 2005, Erik Hovland wrote:

> On Wed, Mar 09, 2005 at 04:12:32PM +0100, Catalin Drula wrote:
> > In addition to the changes strictly related to the HWUART, I have also
> > backported some of the changes made in 2.14.19-rmk7-pxa2 to the irq code
> > and the entry code in arch/arm/kernel/entry-*.S. These were necessary to
> > remove some bugs (for example, some irq numbers were hardcoded instead of
> > being relative to PXA_IRQ_SKIP as they should have been).
>
> I have a question about a change the patch makes to
> include/asm-arm/arch-pxa/irqs.h. There is a little config magic there
> and a line that says:
> #define PXA_IRQ_SKIP 6 /* Only the first 7 IRQs are reserved, but stupid drivers don't like irq 0 so */
>
> Think you could expound on what stupid drivers don't like irq 0. I
> noticed that rmk7-pxa2 has a PXA_IRQ_SKIP of 7 (but they obviously could
> give a flip about h5400 and bluetooth). It would be nice to get the
> stupid drivers fixed instead so it is closer to mainline.

Well, PXA_IRQ_SKIP is the number of irqs that are "reserved" (that is bits
in the beginning of the interrupt register, I forgot what it's called, on
the PXA). On the PXA 250 this number was 8. On the PXA 255 they put the
HWUART irq on bit 7 (previously reserved) so normally we would change
PXA_IRQ_SKIP to 7. That would mean the irq for the HWUART would be 0. The
serial driver (drivers/char/serial.c) uses irq 0 as a reserved value (I
think it means "no irq" or something like that), so intead of changing the
whole driver I opted for the simpler solution (yet a bit hackish, but with
no side effects) of offseting by yet another unit.

The serial driver in 2.4.19 is a real mess in itself since it supports so
many different UARTs. In further kernels (like 2.6) a lot of this stuff
has been broken into smaller pieces and moved under drivers/serial, I
think. The PXA UART also gets its own driver: drivers/serial/pxa.c

Catalin
Received on Fri Mar 11 2005 - 04:30:53 EST