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Message-ID: <bda7328e-eb24-cc60-5be4-a65469f5caa2@arm.com>
Date: Mon, 10 Jul 2017 14:42:20 +0100
From: Vladimir Murzin <vladimir.murzin@....com>
To: Robin Murphy <robin.murphy@....com>,
Christoph Hellwig <hch@....de>,
Vitaly Kuzmichev <vitaly_kuzmichev@...tor.com>
Cc: gregkh@...uxfoundation.org, m.szyprowski@...sung.com,
linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
"George G. Davis" <george_davis@...tor.com>
Subject: Re: [PATCH v2 1/2] drivers: dma-coherent: Fix dev->cma_area vs
dev->dma_mem breakage
On 07/07/17 18:55, Robin Murphy wrote:
> On 07/07/17 17:44, Vladimir Murzin wrote:
>> On 07/07/17 17:06, Robin Murphy wrote:
>>> On 07/07/17 16:40, Vladimir Murzin wrote:
>>>> Christoph,
>>>>
>>>> On 07/07/17 15:27, Christoph Hellwig wrote:
>>>>> Vladimir,
>>>>>
>>>>> this is why I really didn't like overloading the current
>>>>> dma coherent infrastructure with the global pool.
>>>>>
>>>>> And this new patch seems like piling hacks over hacks. I think we
>>>>> should go back and make sure allocations from the global coherent
>>>>> pool are done by the dma ops implementation, and not before calling
>>>>> into them - preferably still reusing the common code for it.
>>>>>
>>>>> Vladimir or Vitaly - can you look into that?
>>>>>
>>>>
>>>> It is really sad that Vitaly and George did not join to discussions earlier,
>>>> so we could avoid being in situation like this.
>>>>
>>>> Likely I'm missing something, but what should happen if device relies on
>>>> dma_contiguous_default_area?
>>>>
>>>> Originally, intention behind dma-default was to simplify things, so instead of
>>>>
>>>> reserved-memory {
>>>> #address-cells = <1>;
>>>> #size-cells = <1>;
>>>> ranges;
>>>>
>>>> coherent_dma: linux,dma {
>>>> compatible = "shared-dma-pool";
>>>> no-map;
>>>> reg = <0x78000000 0x800000>;
>>>> };
>>>> };
>>>>
>>>>
>>>> dev0: dev@...00000 {
>>>> memory-region = <&coherent_dma>;
>>>> /* ... */
>>>> };
>>>>
>>>> dev1: dev@...00000 {
>>>> memory-region = <&coherent_dma>;
>>>> /* ... */
>>>> };
>>>>
>>>> dev2: dev@...00000 {
>>>> memory-region = <&coherent_dma>;
>>>> /* ... */
>>>> };
>>>>
>>>> in device tree we could simply have
>>>>
>>>> reserved-memory {
>>>> #address-cells = <1>;
>>>> #size-cells = <1>;
>>>> ranges;
>>>>
>>>> coherent_dma: linux,dma {
>>>> compatible = "shared-dma-pool";
>>>> no-map;
>>>> reg = <0x78000000 0x800000>;
>>>> linux,dma-default;
>>>> };
>>>> };
>>>>
>>>> and that just work in my (NOMMU) case because there is no CMA there...
>>>>
>>>> However, given that dma-default is being overloaded and there are no device
>>>> tree users merged yet, I would not object stepping back, reverting "drivers:
>>>> dma-coherent: Introduce default DMA pool" and cooperatively rethinking
>>>> design/implementation, so every party gets happy.
>>>
>>> I don't think we need to go that far, I reckon it would be clear enough
>>> to just split the per-device vs. global pool interfaces, something like
>>> I've sketched out below (such that the ops->alloc implementation calls
>>> dma_alloc_from_global_coherent() if dma_alloc_from_contiguous() fails).
>>
>> Would not we need also release and mmap variants?
>
> Sure, that was just bashed out in 2 minutes and diffed into an email on
> the assumption that code would help illustrate the general idea I had in
> mind more clearly than prose alone. I'm certain it won't even compile
> as-is ;)
Ok. I've added missed pieces and even wire-up that with ARM NOMMU and it works
fine for me, but before I go further it'd be handy to know
1. what does Christoph think of that idea?
2. what is Vitaly's use case for dma-default?
Cheers
Vladimir
>
> Robin.
>
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