Re: [PATCH] misc: pci_endpoint_test: Use memcpy_toio()/memcpy_fromio() for BAR tests

[Niklas Cassel <cassel@kernel.org>]

Hello Arnd,

On Tue, Mar 19, 2024 at 08:55:19PM +0100, Arnd Bergmann wrote:
> On Tue, Mar 19, 2024, at 17:53, Niklas Cassel wrote:
> > On Tue, Mar 19, 2024 at 10:18:26PM +0530, Manivannan Sadhasivam wrote:
> >> > 
> >> > I did also see this comment:
> >> > https://github.com/torvalds/linux/blob/master/Documentation/memory-barriers.txt#L2785-L2790
> >> > 
> >> > Do you think that we need to perform any flushing after the memset(),
> >> > to ensure that the data written using memcpy_toio() is actually what
> >> > we expect it to me?
> >> > 
> >> 
> >> The documentation recommends cache flushing only if the normal memory write and
> >> MMIO access are dependent. But here you are just accessing the MMIO. So no
> >> explicit ordering or cache flushing is required.
> >
> > What does dependent mean in this case then?
> >
> > Since the data that we are writing to the device is the data that was
> > just written to memory using memset().
> 
> You need a barrier for the case where the memset() writes to
> a buffer in RAM and then you write the address of that buffer
> into a device register, which triggers a DMA read from the
> buffer. Without a barrier, the side effect of the MMIO write
> may come before the data in the RAM buffer is visible.
> 
> A memcpy_fromio() only involves a single master accessing
> the memory (i.e. the CPU executing both the memset() and the
> memcpy()), so there is no way this can go wrong.

Thank you for the clarification.


Kind regards,
Niklas