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

From: Andy Shevchenko <andriy.shevchenko@intel.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: david@lechnology.com, dri-devel@lists.freedesktop.org,
	linux-spi@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	Jarkko Nikula <jarkko.nikula@linux.intel.com>,
	sam@ravnborg.org
Subject: Re: [PATCH v2 05/11] drm/tinydrm: Remove tinydrm_spi_max_transfer_size()
Date: Wed, 16 Oct 2019 19:13:00 +0300	[thread overview]
Message-ID: <20191016161300.GW32742@smile.fi.intel.com> (raw)
In-Reply-To: <20191015155720.GQ11828@phenom.ffwll.local>

On Tue, Oct 15, 2019 at 05:57:20PM +0200, Daniel Vetter wrote:
> 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?
> > 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.
> > AFAIUI max_dma_len is used when splitting up the buffer for the sg table
> > in spi_map_buf().
> 
> Something like this, as a test patch.

max_transfer_size should be a function. In that case it works.
However I'm not sure it's the best approach, thus, Cc to SPI PXA people.

> diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
> index bb6a14d1ab0f..f77201915033 100644
> --- a/drivers/spi/spi-pxa2xx.c
> +++ b/drivers/spi/spi-pxa2xx.c
> @@ -1707,6 +1707,7 @@ static int pxa2xx_spi_probe(struct platform_device *pdev)
>  		} else {
>  			controller->can_dma = pxa2xx_spi_can_dma;
>  			controller->max_dma_len = MAX_DMA_LEN;
> +			controller->max_transfer_size = MAX_DMA_LEN;
>  		}
>  	}
>  

-- 
With Best Regards,
Andy Shevchenko

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