linux-kernel - Re: [PATCH v2] pci: loongson: Workaround MIPS firmware MRRS settings

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Message-ID: <20230907050805.GA3218@thinkpad>
Date:   Thu, 7 Sep 2023 10:38:05 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     Huacai Chen <chenhuacai@...nel.org>, linux-pci@...r.kernel.org,
        bhelgaas@...gle.com, linux-kernel@...r.kernel.org, kw@...ux.com,
        lpieralisi@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] pci: loongson: Workaround MIPS firmware MRRS settings

On Thu, Sep 07, 2023 at 11:13:00AM +0800, Jiaxun Yang wrote:
> 
> 
> 在 2023/9/7 9:18, Manivannan Sadhasivam 写道:
> [...]
> > Why do you need to walk through every single device instead of just bridges?
> > I'm not the maintainer, but my suggestion is to go for Huacai Chen's solution.
> Hi Mani,
> 
> Thanks for your reply, unfortunately Huacai's solution is impractical in
> this case.
> 
> The problem we have, is firmware (or BIOS) setting improper MRRS for devices
> attached under those bridges. So we have to fix up MRRS for every single
> device.
> We can't iterate child device in bridge quirk because there is no guarantee
> that
> bridge will be probed before  it's child device, partly due to hotplug.
> 

Okay, this clarifies and also warrants improvement in commit message.

You could also use pci_walk_bus() after pci_host_probe() to iterate over the
child devices under root bridge and set MRRS. IMO that would look neat.

- Mani

> This quirk has been in tree for a while, until Huacai refactored it and
> broke some
> systems in 8b3517f88ff2 ("PCI: loongson: Prevent LS7A MRRS increases").
> 
> Also to note that ks_pcie_quirk in drivers/pci/controller/dwc/pci-keystone.c
> uses similar approach.
> > This avoids iterating over bridges/devices two times.
> > 
> > Also, please rename firmware to BIOS, as firmware commonly represents the
> > software running on PCIe endpoint devices.
> Ack, will fix in next reversion.
> 
> Thanks
> - Jiaxun
> > 
> > - Mani
> [...]

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