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

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Message-ID: <c11d2a04-428c-67c2-9cd6-52ce3281cdab@arm.com>
Date:   Wed, 1 Dec 2021 14:18:53 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Lucas Stach <l.stach@...gutronix.de>,
        Michael Walle <michael@...le.cc>,
        etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Cc:     "Lukas F . Hartmann" <lukas@...re.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v2 3/3] drm/etnaviv: use a 32 bit mask as coherent DMA
 mask

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@...gutronix.de>
>>> Signed-off-by: Michael Walle <michael@...le.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().

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.