Re: [PATCH] PCI: rcar: Always allocate MSI addresses in 32bit space

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Wed, Dec 16, 2020 at 06:49:54PM +0100, Marek Vasut wrote:
> On 12/14/20 5:08 PM, Lorenzo Pieralisi wrote:
> > On Sat, Dec 12, 2020 at 08:13:54PM +0100, Marek Vasut wrote:
> > > On 12/10/20 7:11 PM, Lorenzo Pieralisi wrote:
> > > 
> > > [...]
> > > 
> > > > > diff --git a/drivers/pci/controller/pcie-rcar-host.c b/drivers/pci/controller/pcie-rcar-host.c
> > > > > index 1194d5f3341b..ac5c7d7573a6 100644
> > > > > --- a/drivers/pci/controller/pcie-rcar-host.c
> > > > > +++ b/drivers/pci/controller/pcie-rcar-host.c
> > > > > @@ -753,7 +753,7 @@ static int rcar_pcie_enable_msi(struct rcar_pcie_host *host)
> > > > >    	}
> > > > >    	/* setup MSI data target */
> > > > > -	msi->pages = __get_free_pages(GFP_KERNEL, 0);
> > > > > +	msi->pages = __get_free_pages(GFP_KERNEL | GFP_DMA32, 0);
> > > > 
> > > > This does not do what you want on !CONFIG_ZONE_DMA32 (ie arm LPAE).
> > > 
> > > How come? I would expect GFP_DMA32 allocates a buffer below 4 GiB in any
> > > case.
> > 
> > For ARM LPAE allocation falls back to ZONE_NORMAL that happens to work
> > because if there is memory > 4GB it ends up in ZONE_HIGHMEM, so this
> > patch should still work on ARM LPAE too.
> > 
> > Regardless, thoughts above the alternative approach (that saves you
> > a page allocation) ?
> 
> Since this is a bugfix, I would prefer it to be minimal.

Yes, I agree with you on that.

> Also, in case there was some yet undiscovered hardware bug which would
> let the MSI write through, having unused memory as the MSI destination
> address would only lead to write into that memory -- instead of a
> write into some other address.
> 
> Changing this to some hard-coded address (any suggestions?) can be a
> subsequent patch.

The idea was taking the address from the host controller inbound window
(ie an address outside the dma-ranges ~(dma-ranges) and < 4GB), it
should not matter which one. I agree though that this can be a
subsequent patch even though usually we send for -rc* only fixes for
patches that hit the previous merge window - this seems a quite
longstanding (I traced it back to v3.16) one so it would wait till
v5.12, there is time to refactor it.

Thanks,
Lorenzo