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

[Lucas Stach <l.stach@pengutronix.de>]

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@pengutronix.de>
> > Signed-off-by: Michael Walle <michael@walle.cc>
> > ---
> >   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().

Regards,
Lucas

> Robin.
> 
> > +		dev_dbg(&pdev->dev, "No suitable DMA available\n");
> > +		return -ENODEV;
> > +	}
> > +
> >   	/*
> >   	 * Apply the same DMA configuration to the virtual etnaviv
> >   	 * device as the GPU we found. This assumes that all Vivante
> > @@ -671,8 +689,6 @@ static int __init etnaviv_init(void)
> >   			of_node_put(np);
> >   			goto unregister_platform_driver;
> >   		}
> > -		pdev->dev.coherent_dma_mask = DMA_BIT_MASK(40);
> > -		pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
> >   
> >   		ret = platform_device_add(pdev);
> >   		if (ret) {
> >