linux-kernel - Re: [RFC PATCH] xen/gntdev: Stop abusing DT of_dma

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Date:   Wed, 2 Oct 2019 04:42:59 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        Rob Herring <robh@...nel.org>
CC:     Robin Murphy <robin.murphy@....com>,
        Julien Grall <julien.grall@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Juergen Gross <jgross@...e.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Oleksandr Andrushchenko <andr2000@...il.com>
Subject: Re: [RFC PATCH] xen/gntdev: Stop abusing DT of_dma_configure API

On 10/1/19 9:23 PM, Stefano Stabellini wrote:
> On Thu, 26 Sep 2019, Rob Herring wrote:
>> On Thu, Sep 26, 2019 at 6:16 AM Oleksandr Andrushchenko
>> <Oleksandr_Andrushchenko@...m.com> wrote:
>>> On 9/26/19 1:46 PM, Robin Murphy wrote:
>>>> On 2019-09-26 11:17 am, Oleksandr Andrushchenko wrote:
>>>>> On 9/26/19 12:49 PM, Julien Grall wrote:
>>>>>> Hi Rob,
>>>>>>
>>>>>>
>>>>>> On 9/25/19 10:50 PM, Rob Herring wrote:
>>>>>>> As the comment says, this isn't a DT based device. of_dma_configure()
>>>>>>> is going to stop allowing a NULL DT node, so this needs to be fixed.
>>>>>> And this can't work on arch not selecting CONFIG_OF and can select
>>>>>> CONFIG_XEN_GRANT_DMA_ALLOC.
>>>>>>
>>>>>> We are lucky enough on x86 because, AFAICT, arch_setup_dma_ops is just
>>>>>> a nop.
>>>>>>
>>>>> No luck is needed as [1] does nothing for those platforms not using
>>>>> CONFIG_OF
>>>>>>> Not sure exactly what setup besides arch_setup_dma_ops is needed...
>>>>>> We probably want to update dma_mask, coherent_dma_mask and
>>>>>> dma_pfn_offset.
>>>>>>
>>>>>> Also, while look at of_configure_dma, I noticed that we consider the
>>>>>> DMA will not be coherent for the grant-table. Oleksandr, do you know
>>>>>> why they can't be coherent?
>>>>> The main and the only reason to use of_configure_dma is that if we don't
>>>>> then we
>>>>> are about to stay with dma_dummy_ops [2]. It effectively means that
>>>>> operations on dma-bufs
>>>>> will end up returning errors, like [3], [4], thus not making it possible
>>>>> for Xen PV DRM and DMA
>>>>> part of gntdev driver to do what we need (dma-bufs in our use-cases
>>>>> allow zero-copying
>>>>> while using graphics buffers and many more).
>>>>>
>>>>> I didn't find any better way of achieving that, but of_configure_dma...
>>>>> If there is any better solution which will not break the existing
>>>>> functionality then
>>>>> I will definitely change the drivers so we do not abuse DT )
>>>>> Before that, please keep in mind that merging this RFC will break Xen PV
>>>>> DRM +
>>>>> DMA buf support in gntdev...
>>>>> Hope we can work out some acceptable solution, so everyone is happy
>>>> As I mentioned elsewhere, the recent dma-direct rework means that
>>>> dma_dummy_ops are now only explicitly installed for the ACPI error
>>>> case, so - much as I may dislike it - you should get regular
>>>> (direct/SWIOTLB) ops by default again.
>>> Ah, my bad, I missed that change. So, if no dummy dma ops are to be used
>>> then
>>> I believe we can apply both changes, e.g. remove of_dma_configure from
>>> both of the drivers.
>> What about the dma masks? I think there's a default setup, but it is
>> considered a driver bug to not set its mask. xen_drm_front sets the
>> coherent_dma_mask (why only 32-bits though?), but not the dma_mask.
>> gntdev is doing neither. I could copy out what of_dma_configure does
>> but better for the Xen folks to decide what is needed or not and test
>> the change. I'm not setup to test any of this.
> FYI I have seen the issue Oleksandr is talking about too. I confirm that
> the only reason for the of_configure_dma call is to get away from the
> dummy_dma_ops and use the default dma_ops instead. I think this should
> be mentioned in the commit message so that if one day the behavior
> regarding dummy_dma_ops changes one more time, hopefully we'll be able
> to figure out the issue more easily with bisection.
>
> In regards to the coherent_dma_mask and dma_mask, I can't see why gntdev
> would have any dma addressing limitations, so we should be able to set
> both to 64 bits.  I also can't see why xen_drm_front would limit it to
> 32 bits, after all this is just the frontend, if anything it would be
> the backend that has a limitation. So, we should be able to set both
> dma_mask and coherent_dma_mask in xen_drm_front to 64 bits. Oleksandr,
> can you confirm?
I am totally fine with 64-bits in both cases and
agree with what Stefano says.