Re: [PATCH v4 01/16] spi: dw: Add Tx/Rx finish wait methods to the MID DMA

From: Serge Semin
Date: Mon May 25 2020 - 17:36:31 EST

On Mon, May 25, 2020 at 12:41:32PM +0100, Mark Brown wrote:
> On Sat, May 23, 2020 at 11:34:10AM +0300, Serge Semin wrote:
> > On Fri, May 22, 2020 at 04:22:41PM +0100, Mark Brown wrote:
> > > Right, that definitely needs to be fixed then - 8MHz is indeed a totally
> > > normal clock rate for SPI so people will hit it. I guess if there's a
> > > noticable performance hit to defer to thread then we could implement
> > > both and look at how long the delay is going to be to decide which to
> > > use, that's annoyingly complicated though so if the overhead is small
> > > enough we could just not bother.
> > As I suggested before we can implement a solution without performance drop.
> > Just wait for the DMA completion locally in the dw_spi_dma_transfer() method and
> > return 0 instead of 1 from the transfer_one() callback. In that function we'll
> > wait while DMA finishes its business, after that we can check the Tx/Rx FIFO
> > emptiness and wait for the data to be completely transferred with delays or
> > sleeps or whatever.
> No extra context switches there at least, that's the main issue.

Right. There won't be extra context switch.

> > NOTE Currently the DW APB SSI driver doesn't set xfer->effective_speed_hz, though as
> > far as I can see that field exists there to be initialized by the SPI controller
> > driver, right? If so, strange it isn't done in any SPI drivers...
> Yes. Not that many people are concerned about the exact timing it turns
> out, the work that was being used for never fully made it upstream.
> > What do think about this?
> Sure.

Great. I'll send a new patchset soon. It'll fix the Tx/Rx non-empty issue in
accordance with the proposed design.


> > patchset "spi: dw: Add generic DW DMA controller support" (it's being under
> > review in this email thread) ? Anyway, if the fixup is getting to be that
> > complicated, will it have to be backported to another stable kernels?
> No, if it's too invasive it shouldn't be (though the stable people might
> decide they want it anyway these days :/ ).