[PATCH v2 05/11] drm/tinydrm: Remove tinydrm_spi_max_transfer_size()

[Andy Shevchenko]

On Tue, Oct 15, 2019 at 05:41:53PM +0200, Noralf Trønnes wrote:
> Den 15.10.2019 16.32, skrev Andy Shevchenko:
> > On Fri, Jul 19, 2019 at 05:59:10PM +0200, Noralf Trønnes wrote:
> >> spi-bcm2835 can handle >64kB buffers now so there is no need to check
> >> ->max_dma_len. The tinydrm_spi_max_transfer_size() max_len argument is
> >> not used by any callers, so not needed.
> >>
> >> Then we have the spi_max module parameter. It was added because
> >> staging/fbtft has support for it and there was a report that someone used
> >> it to set a small buffer size to avoid popping on a USB soundcard on a
> >> Raspberry Pi. In hindsight it shouldn't have been added, I should have
> >> waited for it to become a problem first. I don't know it anyone is
> >> actually using it, but since tinydrm_spi_transfer() is being moved to
> >> mipi-dbi, I'm taking the opportunity to remove it. I'll add it back to
> >> mipi-dbi if someone complains.
> >>
> >> With that out of the way, spi_max_transfer_size() can be used instead.
> >>
> >> The chosen 16kB buffer size for Type C Option 1 (9-bit) interface is
> >> somewhat arbitrary, but a bigger buffer will have a miniscule impact on
> >> transfer speed, so it's probably fine.
> > 
> > This breaks the SPI PXA2xx case I'm using. The world is not a Pi:e.
> > 
> > [  388.445752] mi0283qt spi-PRP0001:01: DMA disabled for transfer length 153600 greater than 65536
> > [  388.634437] mi0283qt spi-PRP0001:01: DMA disabled for transfer length 153600 greater than 65536
> > [  388.822933] mi0283qt spi-PRP0001:01: DMA disabled for transfer length 153600 greater than 65536
> > 
> > The crucial thing is to check the transfer size against maximum DMA length
> > of the master.
> > 
> 
> Isn't this a spi controller driver problem?

It doesn't matter. This patch made a regression. Before it worked,
now it doesn't.

> spi_max_transfer_size() tells the client what the maximum transfer
> length is. The controller driver can use ctlr->max_transfer_size if it
> has restrictions.

It might be a problem of the SPI core.

> AFAIUI max_dma_len is used when splitting up the buffer for the sg table
> in spi_map_buf().

> > Please, fix.

Should I send the revert?

-- 
With Best Regards,
Andy Shevchenko