Re: [PATCH 1/2] dma: add Qualcomm Technologies HIDMA management driver

From: Andy Shevchenko
Date: Fri Oct 30 2015 - 14:18:56 EST

On Fri, Oct 30, 2015 at 8:08 PM, Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote:

>> The ACPI tables DSDT and CSRT (more info here:
>> defines properties.
>> DSDT:
>> per DMAC: the resources
>> per client: FixedDMA descriptor that contains channel / request line
>> pair.
>> CSRT:
>> necessary table to map which DMAC provides which request line, thus
>> request line numbering are global on platform.
>> When DMAC driver is probed in the running system it should call as
>> well registration function from acpi-dma.c.
>> All clients when use new DMA slave API gets channel automatically
>> based on their FixedDMA property.
>> So, above is how it should be done. Didn't actually checked what this
>> driver does.
> I was going to reply to all the questions in one pass but let me handle
> piece by piece.
> Here are some facts.
> - This hardware supports memcpy and memset only.
> - Memset feature was removed from the kernel sometime around 3.14. So no
> memset support in this driver either.
> - The hardware does not support DMA slave support
> - The goal is to provide an interface to DMA engine framework for memcpy
> optimization so that the rest of the kernel drivers and applications make
> use of the hardware.

> CSRT is an Intel specific ACPI table for slave devices.

It was designed by Microsoft to support multiple controllers, in
particular DMACs.
Have you read that document I posted link to?

> It was decided by
> Linaro that CSRT will not be supported for ARM64.

Interesting, ARM64 platforms are not going to have more than one DMAC
per system?

> There were some discussions in ACPI forums to define a similar table for
> ARM64 but we are not there today and this hardware does not support slave
> interface.
> ACPI enumeration is just like any other platform device. The driver gets
> looked up by a QCOM specific HID and the driver gets probed with the rest of
> the arguments in DSM object similar to device-tree attributes. The code uses
> device functions so the driver is not aware of where the parameters are
> coming from.

With Best Regards,
Andy Shevchenko
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