Re: [PATCH v4 2/2] PCI: iproc: Add outbound configuration for 32-bit I/O region

[Lorenzo Pieralisi]

On Wed, Apr 03, 2019 at 08:41:44AM +0530, Srinath Mannam wrote:
> Hi Lorenzo,
> 
> Please see my reply below,
> 
> On Tue, Apr 2, 2019 at 7:08 PM Lorenzo Pieralisi
> <lorenzo.pieralisi@xxxxxxx> wrote:
> >
> > On Tue, Apr 02, 2019 at 04:16:13PM +0530, Srinath Mannam wrote:
> >
> > [...]
> >
> > > > Ok - I start to understand. What does it mean in HW terms that your
> > > > 32bit AXI address region size is 32MB ? Please explain to me in details.
> > > >
> > > In our PCIe controller HW, AXI address from 0x42000000 to 0x44000000
> > > of 32MB size and .
> > > AXI address from 0x400000000 to 0x480000000 of 2GB size are provided
> > > to map ob address.
> > > First IO region is inside 32bit address and second IO region is
> > > outside 32bit address.
> > > This code change is to map first IO region(0x42000000 to 0x44000000).
> > >
> > > > IIUC you are using an OARR0 of 128MB in size to map a 32MB address
> > > > region, that's what I understand this patch does (and the lowest index
> > > > corresponds to the smallest possible size - it is far from clear by
> > > > looking at the patch).
> > > Yes, lowest index corresponds to smallest possible size (128MB).
> > > In our controller we have multiple windows like OARR0, OARR1, OARR2,
> > > OARR3 all supports multiple sizes from 128MB to 1024MB.
> > > These details are given at the top of this driver file, as shown
> > > below. all windows supports 128MB size still we must use OARR0 window
> > > to configure first IO region(0x42000000 to 0x44000000).
> > >
> > > static const struct iproc_pcie_ob_map paxb_v2_ob_map[] = {
> > >         {
> > >                 /* OARR0/OMAP0 */
> > >                 .window_sizes = { 128, 256 },
> > >                 .nr_sizes = 2,
> > >         },
> > >         {
> > >                 /* OARR1/OMAP1 */
> > >                 .window_sizes = { 128, 256 },
> > >                 .nr_sizes = 2,
> > >         },
> > >         {
> > >                 /* OARR2/OMAP2 */
> > >                 .window_sizes = { 128, 256, 512, 1024 },
> > >                 .nr_sizes = 4,
> > >         },
> > >         {
> > >                 /* OARR3/OMAP3 */
> > >                 .window_sizes = { 128, 256, 512, 1024 },
> > >                 .nr_sizes = 4,
> > >         },
> > > };
> >
> > Ok so this patch allows mapping an AXI I/O window that is smaller
> > than OARR possible sizes, why it was not done from the beginning
> > I really do not know.
> >
> Same Iproc driver we use for multiple SOCs, in previous SOCs does not
> have 32-bit AXI address region to map ob.
> In the present SOC, 32-bit AXI address region is available so that
> this change is added.
> 
> > Now explain this to me please:
> >
> > > This patch add outbound window configuration to map below 32-bit I/O range
> > > with corresponding PCI memory, which helps to access I/O region in ARM
> > > 32-bit and one to one mapping of I/O region to PCI memory.
> > >
> > > Ex:
> > > 1. ranges DT property given for current driver is,
> > >     ranges = <0x83000000 0x0 0x40000000 0x4 0x00000000 0 0x40000000>;
> > >     I/O region address is 0x400000000
> > > 2. ranges DT property can be given after this patch,
> > >     ranges = <0x83000000 0x0 0x42000000 0x0 0x42000000 0 0x2000000>;
> > >     I/O region address is 0x42000000
> >
> > Why 1:1 AXI<->PCI address mapping is not possible in (1), how does the
> > current code works on 32-bit systems and what's the benefit your change
> > is bringing.
> non-prefetchable memory range can only support 32-bit addresses, so
> that we have taken 32-bit PCI bus address in (1).
> current code does not work in 32-bit systems. In the present SOC with
> this new change we can access from 32-bit CPU.

Thank you. I rewrote the log and pushed patches to pci/iproc, please
have a look (Ray/Scott please do have a look too) and report back
if that's fine.

Do you agree that the initial commit was lacking _significant_
information ? Please remember that the commit log plays a fundamental
part in understanding a change and this one is a very important one
so I am being pedantic on it.

Thanks,
Lorenzo