Hi Arnd, Miquel

On Thu, Sep 22, 2022, at 11:36 AM, Miquel Raynal wrote:

vkorenblit@sequans.com wrote on Thu, 22 Sep 2022 10:18:46 +0200:

Correct, this was my initial idea. However, this driver should work
with every architecture or do we limit the scope to arm/arm64/x86_64?

The driver should work on ARM and aarch64, I'm not aware of other
architectures with this IP.

The driver should compile when COMPILE_TEST=y.

It should also be written in a way that makes it plausible to
use elsewhere. Since this is just a licensed IP core, there is
a good chance that someone reused it on mips or riscv, or
anything else.

I believe what Valentin wanted to achieve in the first place, was to
use 64-bit accesses when relevant (otherwise it does not work).  

The width is read from a device specific register at
runtime, it is not related to the architecture you are
running on, presumably this is hardwired during the
design of an SoC, based on the capabilities of the DMA
engine:

Well, yes, but in the mean time 64-bit DMA width will never be
used on 32-bit platforms.

Why? Most architectures (including x86 and arm) allow you to
run a 32-bit kernel on a 64-bit SoC. While this is almost always
a bad idea to actually do, a driver should be written to
work correctly in this setup.

This usually means the largest access that is valid for
reading from the FIFO, but usually smaller accesses work
as well, just slower.  

Mmh, ok, that's interesting, thanks for the pointer.

But in the mean time I am only half satisfied, because we plan to do
twice more accesses than needed _just_ because of a the COMPILE_TEST
constraint.

In my example, I had an #ifdef so it would only fall back
to 32-bit accesses on the 64-bit register when running an
actual 32-bit kernel, but leaving the 64-bit case efficient.

Sorry for my late reply. I've just tested this and unfortunately
the two sequential 32-bit accesses (with OFF0==0 and OFF1==4 seem
to trigger sdma_err. I need to check some waveforms to verify if it 
happens right after the first access. 

    Arnd

Best regards,

Valentin