linux-kernel - Re: [PATCH] PCI: dw-rockchip: Configure max payload size on host init

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Message-ID: <8af8a418-b808-4a14-9969-1c6d549e1633@163.com>
Date: Sat, 19 Apr 2025 01:21:54 +0800
From: Hans Zhang <18255117159@....com>
To: Niklas Cassel <cassel@...nel.org>, Bjorn Helgaas <helgaas@...nel.org>
Cc: Shawn Lin <shawn.lin@...k-chips.com>, lpieralisi@...nel.org,
 kw@...ux.com, bhelgaas@...gle.com, heiko@...ech.de,
 manivannan.sadhasivam@...aro.org, robh@...nel.org, jingoohan1@...il.com,
 thomas.richard@...tlin.com, linux-pci@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH] PCI: dw-rockchip: Configure max payload size on host init



On 2025/4/18 22:55, Niklas Cassel wrote:
> 
> 
> On 18 April 2025 14:33:08 CEST, Hans Zhang <18255117159@....com> wrote:
>>
>> Dear Bjorn,
>>
>> Thanks your for reply. Niklas and I attempted to modify the relevant logic in drivers/pci/probe.c and found that there was a lot of code judging the global variable pcie_bus_config. At present, there is no good method. I will keep trying.
>>
>> I wonder if you have any good suggestions? It seems that the code logic regarding pcie_bus_config is a little complicated and cannot be modified for the time being?
> 
> 
> Hans,
> 
> If:
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 364fa2a514f8..2e1c92fdd577 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2983,6 +2983,13 @@ void pcie_bus_configure_settings(struct pci_bus *bus)
>           if (!pci_is_pcie(bus->self))
>                   return;
>    +       /*
> +        * Start off with DevCtl.MPS == DevCap.MPS, unless PCIE_BUS_TUNE_OFF.
> +        * This might get overriden by a MPS strategy below.
> +        */
> +       if (pcie_bus_config != PCIE_BUS_TUNE_OFF)
> +               smpss = pcie_get_mps(bus->self);
> +

Dear Niklas,

Thank you very much for your reply and thoughts.

pcie_get_mps: Returns maximum payload size in bytes

I guess you want to obtain the DevCap MPS. But the purpose of the smpss 
variable is to save the DevCtl MPS.

>           /*
>            * FIXME - Peer to peer DMA is possible, though the endpoint would need
>            * to be aware of the MPS of the destination.  To work around this,
> 
> 
> 
> does not work, can't you modify the code slightly so that it works?
> 
> I haven't tried myself, but considering that it works when walking the bus, it seems that it should be possible to get something working.
> 


After making the following modifications, my test shows that it is 
normal. If the consideration is not comprehensive. Could Bjorn and 
Niklas please review my revisions?

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 364fa2a514f8..5b54f1b0a91d 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2951,8 +2951,7 @@ static int pcie_bus_configure_set(struct pci_dev 
*dev, void *data)
         if (!pci_is_pcie(dev))
                 return 0;

-       if (pcie_bus_config == PCIE_BUS_TUNE_OFF ||
-           pcie_bus_config == PCIE_BUS_DEFAULT)
+       if (pcie_bus_config == PCIE_BUS_TUNE_OFF)
                 return 0;

         mps = 128 << *(u8 *)data;
@@ -2991,7 +2990,8 @@ void pcie_bus_configure_settings(struct pci_bus *bus)
         if (pcie_bus_config == PCIE_BUS_PEER2PEER)
                 smpss = 0;

-       if (pcie_bus_config == PCIE_BUS_SAFE) {
+       if ((pcie_bus_config == PCIE_BUS_SAFE) ||
+           (pcie_bus_config != PCIE_BUS_TUNE_OFF)) {
                 smpss = bus->self->pcie_mpss;

                 pcie_find_smpss(bus->self, &smpss);


Best regards,
Hans