linux-kernel - Re: [PATCH] misc: sram: Add dma-heap-export reserved SRAM area type

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Date:   Wed, 29 Apr 2020 15:05:41 -0400
From:   "Andrew F. Davis" <afd@...com>
To:     Rob Herring <robh+dt@...nel.org>
CC:     Sumit Semwal <sumit.semwal@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        <linaro-mm-sig@...ts.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] misc: sram: Add dma-heap-export reserved SRAM area type

On 4/27/20 11:17 AM, Rob Herring wrote:
> On Fri, Apr 24, 2020 at 5:27 PM Andrew F. Davis <afd@...com> wrote:
>>
>> This new export type exposes to userspace the SRAM area as a DMA-Heap,
>> this allows for allocations as DMA-BUFs that can be consumed by various
>> DMA-BUF supporting devices.
>>
>> Signed-off-by: Andrew F. Davis <afd@...com>
>> ---
>>  .../devicetree/bindings/sram/sram.yaml        |   8 +
> 
> Separate patch and needs to go to DT list...
> 


Okay, will split for v2.


>>  drivers/misc/Kconfig                          |   7 +
>>  drivers/misc/Makefile                         |   1 +
>>  drivers/misc/sram-dma-heap.c                  | 243 ++++++++++++++++++
>>  drivers/misc/sram.c                           |  20 +-
>>  drivers/misc/sram.h                           |  17 ++
>>  6 files changed, 292 insertions(+), 4 deletions(-)
>>  create mode 100644 drivers/misc/sram-dma-heap.c
>>
>> diff --git a/Documentation/devicetree/bindings/sram/sram.yaml b/Documentation/devicetree/bindings/sram/sram.yaml
>> index 7b83cc6c9bfa..b8e33c8d205d 100644
>> --- a/Documentation/devicetree/bindings/sram/sram.yaml
>> +++ b/Documentation/devicetree/bindings/sram/sram.yaml
>> @@ -105,6 +105,14 @@ patternProperties:
>>            manipulation of the page attributes.
>>          type: boolean
>>
>> +      dma-heap-export:
>> +        description:
>> +          Similar to 'pool' and 'export' this region will be exported for use
>> +          by drivers, devices, and userspace using the DMA-Heaps framework.
>> +          NOTE: This region must be page aligned on start and end in order to
>> +          properly allow manipulation of the page attributes.
>> +        type: boolean
> 
> Though I'm not sure this should be in DT. You have to change your
> firmware to enable a new kernel feature? We also already have 'export'
> which sounds like the same function. Or 'pool' though reading the
> description, I don't really understand it's use.
> 


Maybe I could just re-use 'export', right now that property causes the
SRAM region to be exported to userspace as a file one can read/write.
Exporting via dma-heaps/dma-buf allows more flexibility as one can pass
the exported regions to other devices or mmap them directly in userspace.

It's up to the SRAM driver maintainer if re-purposing that property in
the driver is acceptable.

'Pool' doesn't make much sense to me either, it creates a gen-pool out
of the region, but I cant find any users not in combination with the
other properties.


> What combination of all 3 of these options would be valid?
> 


>From looking at the SRAM driver, it seems each of the option must be
used exclusively per region, combining will cause issues. I can update
the documentation for the same after we settle on a strategy for this
new property.

Andrew


> Rob
>