Re: [PATCH v3] PCI: rcar-gen4: Limit Max_Read_Request_Size and Max_Payload_Size to 256 Bytes

[Marek Vasut]

On 9/6/26 7:07 PM, Rong Zhang wrote:

Hello Rong,

I apologize for my delayed reply.

> > > If you need more information I can do some more experiments with the PCIe
> > > switch card.
> >
> > Please look at the commit message [1] , that lists the exact test and
> > the PCIe NVMe SSD which triggers the data corruption (let me know if
> > something is unclear). I think any PCIe NVMe SSD which is non-HMB with
> > MRRS on the SSD size that is > MRRS on the controller side will trigger
> > this issue too.
>
> Unfortunately I don't have any non-HMB NVMe SSD, and it's unlikely I'll
> get one anytime soon due to the crazy price. Before the AI supply chain
> crisis, I bought several NVMe SSDs, but I chose HMB ones as they generate
> less heat and consume less electricity.

Maybe this could be reproduced even with another PCIe device with 
MRRS=512 or more, which posts MRd to the PCIe controller with MRRS=128 
or 256. The AHCI controller might be capable of that. I only have the 
SSD as a reproducer, but there could easily be more.

> > If you could reproduce it on the Loongson, then yes, it
> > is affected. You might have to disable the current MRRS workaround in
> > the Loongson PCIe driver though as it might hide the issue.
>
> Due to the lack of non-HMB NVMe SSDs, I can't reproduce the exact same
> issue. Theoretically I may try to mess around with the AHCI controller
> (see the mentioned commit below) by manually overriding MRRS, as my
> device's firmware already clamps it. We will see...

Does the AHCI controller have MRRS higher than the PCIe controller ?

> The firmware of many Loongson devices (except for some MIPS-based ones,
> see the commit message of commit ef61a0405742 ("PCI: loongson: Limit MRRS
> to 256")) already clamps MRRS for most PCIe devices, which has proven
> that the 7A1000/7A2000 chipset does need the MRRS quirk.

This still only applies the MRRS limit to the PCIe controller, not to 
all devices on the bus, correct ?

I would like to find out whether all the devices (not only the 
controller) on the PCIe bus on Loongson must have MRRS limited or not.
-> If yes, then I should start preparing core patches
-> If no, then this quirk might specific to this controller

> Therefore, you
> can just go ahead with preparing the core patches without the need of
> waiting for my test result. It seems that your approach is not going to
> change the current fixup effects in a significant way, so it should be
> fine.

[...]

Thank you for your help !

--
Best regards,
Marek Vasut