Re: [PATCH v2 3/3] drm/etnaviv: use a 32 bit mask as coherent DMA mask

[Robin Murphy]

On 2021-12-01 13:41, Lucas Stach wrote:
> Hi Robin,
>
> Am Mittwoch, dem 01.12.2021 um 12:50 +0000 schrieb Robin Murphy:
> > Sorry I missed this earlier...
> >
> > On 2021-09-07 17:49, Michael Walle wrote:
> > > The STLB and the first command buffer (which is used to set up the TLBs)
> > > has a 32 bit size restriction in hardware. There seems to be no way to
> > > specify addresses larger than 32 bit. Keep it simple and restict the
> > > addresses to the lower 4 GiB range for all coherent DMA memory
> > > allocations.
> > >
> > > Please note, that platform_device_alloc() will initialize dev->dma_mask
> > > to point to pdev->platform_dma_mask, thus dma_set_mask() will work as
> > > expected.
> > >
> > > While at it, move the dma_mask setup code to the of_dma_configure() to
> > > keep all the DMA setup code next to each other.
> > >
> > > Suggested-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> > > Signed-off-by: Michael Walle <michael@xxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/etnaviv/etnaviv_drv.c | 20 ++++++++++++++++++--
> > >    1 file changed, 18 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.c b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
> > > index 54eb653ca295..0b756ecb1bc2 100644
> > > --- a/drivers/gpu/drm/etnaviv/etnaviv_drv.c
> > > +++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
> > > @@ -613,6 +613,24 @@ static int etnaviv_pdev_probe(struct platform_device *pdev)
> > >    			component_match_add(dev, &match, compare_str, names[i]);
> > >    	}
> > >    
> > > +	/*
> > > +	 * PTA and MTLB can have 40 bit base addresses, but
> > > +	 * unfortunately, an entry in the MTLB can only point to a
> > > +	 * 32 bit base address of a STLB. Moreover, to initialize the
> > > +	 * MMU we need a command buffer with a 32 bit address because
> > > +	 * without an MMU there is only an indentity mapping between
> > > +	 * the internal 32 bit addresses and the bus addresses.
> > > +	 *
> > > +	 * To make things easy, we set the dma_coherent_mask to 32
> > > +	 * bit to make sure we are allocating the command buffers and
> > > +	 * TLBs in the lower 4 GiB address space.
> > > +	 */
> > > +	if (dma_set_mask(&pdev->dev, DMA_BIT_MASK(40)) ||
> > > +	    dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32))) {
> >
> > Since AFAICS you're not changing the default dma_mask pointer to point
> > to some storage other than the coherent mask, the dma_set_mask() call
> > effectively does nothing and both masks will end up reading back as 32 bits.
>  From what I can see the dma_mask has allocated storage in the platform
> device and does not point to the coherent dma mask, see
> setup_pdev_dma_masks().

Urgh, apologies for the confusion - seems I had one of those mental 
short-circuits and was utterly convinced that that's what the platform 
device setup did, but of course it's only the fallback case in 
of_dma_configure(). Sorry for the noise!

Cheers,
Robin.