Re: [edk2-devel] [edk2-platforms][PATCH V1 07/11] Platform/ARM/Morello: Port PCI Express library

["Ard Biesheuvel" <ardb@kernel.org>]

On Mon, 13 Dec 2021 at 13:41, chandni cherukuri <ch.chandni@gmail.com> wrote:
>
> Hi Ard,
>
> Thanks for the response.
>
> I had a look at the Synquacer platform where there is a single custom
> PCIe library implemented.
> So, is it better to implement a single custom PCIe library instead of
> two custom PCIe libraries?
>

Yes, for two reasons:
- there is prior art for this solution, and solving the same problem
in two different ways in the same codebase is bad form;
- those two libraries are layered one on top of the other, which means
that your fix requires changes to two different layers, making the
interface between those two layers obviously unfit to represent the
hardware in question.


> If that is the case then will implement a single custom PCIe library
> for Morello.
>
> Thanks
> Chandni
> On Fri, Dec 10, 2021 at 4:12 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > On Thu, 9 Dec 2021 at 13:30, chandni cherukuri <ch.chandni@gmail.com> wrote:
> > >
> > > Hi Ard, Leif, Sami, Pierre,
> > >
> > > For Morello platform, we created two custom libraries based on the
> > > below two native libraries:
> > > -          https://github.com/tianocore/edk2/tree/master/MdePkg/Library/BasePciSegmentLibPci
> > > -          https://github.com/tianocore/edk2/tree/master/MdePkg/Library/BasePciExpressLib
> > >
> > > because of the following reasons.
> > > 1.       In Morello platform, the ECAM region of the two root ports
> > > are placed in non-contiguous memory as per the memory map architecture
> > > of the Morello platform. The native PCI Express Library expects both
> > > the ECAM regions for all the root ports to be contiguous.
> >
> > Do you mean root ports or host bridges? If root ports don't share the
> > MMIO resource windows provided by the respective host bridges, they
> > are really different host bridges and should be modeled as such.
> >
> > Note that the Cadence and Synopsys IP usually does not implement a
> > root port at all, and gets away with this because it only implements a
> > single link.
> >
> > > 2.       IORT and kernel currently require each root port to be in a
> > > separate segment. You can refer to the code for more details -
> > >
> >
> > Please take a look at the SynQuaecer platform for inspiration. That
> > platform is very similar in this respect.
> >
> >  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/acpi/arm64/iort.c#n307.
> > > The native PCI Segment Library supports only a single segment.
> > >
> > > Because of these reasons, the current patch series adapts the native
> > > libraries as follows:
> > >
> > > The custom PCI Segment Library passes the complete address which is
> > > consumed by the custom PCI Express library where based on the Segment
> > > number, the base address of the PCI Express is returned.
> > >
> > > The rationale behind maintaining two separate libraries is that when
> > > the software evolves and support for multiple segments is adapted in
> > > the native PCI Segment Library the custom library could be removed.
> > > Also, we might have other protocols which might try to use the PCI
> > > Express library directly. However, there are some other platforms
> > > where all the platform specific changes have gone in a single custom
> > > PCI library
> > >
> > > Could you please provide some inputs as to which of two approaches
> > > would be better to follow as there is one more platform to follow
> > > based on the decision taken?
> > >
> > > Thanks
> > > Chandni
> > >
> > > On Wed, Dec 8, 2021 at 8:25 AM Khasim Mohammed <khasim.mohammed@arm.com> wrote:
> > > >
> > > > Hi Sami, Chandni,
> > > >
> > > > There was a suggestion from Pierre on a similar patch for N1SDP to remove PCIExpressLib.c and move to workarounds to PCISegmentLib.c,
> > > >
> > > > https://edk2.groups.io/g/devel/message/84165?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2Ckhasim%2C20%2C2%2C0%2C87257273
> > > >
> > > > I think we should discuss this implementation for both N1SDP and Morello as they have similar implementation.
> > > >
> > > > Regards,
> > > > Khasim
> > > >