[RFC] libertas: remove TX queues from GSPI driver

[Andrey Yurovsky]

On Mon, Jun 22, 2009 at 1:04 AM, Sebastian Andrzej
Siewior<sebatian at breakpoint.cc> wrote:
> * Andrey Yurovsky | 2009-06-19 15:39:28 [-0700]:
>
>>Posting this as an RFC in case others want to help test it.  This
>>work-in-progress patch removes the redundant command and data TX queues
>>from the Libertas GSPI driver, it also therefore removes a memcpy from
>>if_spi_host_to_card and the locking mechanisms for the queues in
>>question.  The goal is to improve performance and reduce overhead by
>>simplifying the TX path.
>>
>>The driver works after this patch, however IEEE PS is somewhat broken
>>(especially temporarily waking the device to issue commands).  This is
>>caused, in part, by the IRQ handler preempting the libertas main thread
>>(for example, when the main thread calls if_spi_host_to_card and an IRQ
>>comes in) so we need a synchronization solution that allows interrupt
>>servicing to be done when it's safe to do so, that is we need to lock
>>the bus properly.
>
> Why is it now okay now to block in if_spi_host_to_card()? Was this never
> an issue?

We don't really block, if_spi_host_to_card is called by the main
thread (synchronized by the dnld_sent flag, which gets cleared by the
IRQ handler) and it writes out the command or TX packet directly and
then returns.  The issue is that you can't sleep in a real IRQ
handler, like the GPIO IRQ that we have in this driver.

> Would it help if you replace request_irq() with request_threaded_irq()?
> With that change it should similar to other interfaces since you can
> read from the chip within the interrupt handler.

It's an actual IRQ handler that takes care of a GPIO interrupt.  This
is different from, say, SDIO, where you use a bus interrupt that the
subsystem provides you.  We can never sleep in that handler, hence the
additional thread.  Almost any implementation of the underlying SPI
controller will sleep at some point, so we can't check, for example,
the IRQ status register right in the handler.

  -Andrey