linux-kernel - Re: [PATCH] PCI: endpoint: pci-epf-test: Limit PCIe BAR size for fixed BARs

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Message-ID: <1jjz2d4a5f.fsf@starbuckisacylon.baylibre.com>
Date: Fri, 05 Sep 2025 09:43:24 +0200
From: Jerome Brunet <jbrunet@...libre.com>
To: Marek Vasut <marek.vasut@...lbox.org>
Cc: Niklas Cassel <cassel@...nel.org>,  Damien Le Moal <dlemoal@...nel.org>,
  Marek Vasut <marek.vasut+renesas@...lbox.org>,
  linux-pci@...r.kernel.org,  Krzysztof Wilczyński
 <kwilczynski@...nel.org>,  Bjorn Helgaas <bhelgaas@...gle.com>,  Frank Li
 <Frank.Li@....com>,  Kishon Vijay Abraham I <kishon@...nel.org>,
  Manivannan Sadhasivam <mani@...nel.org>,  Wang Jiang
 <jiangwang@...inos.cn>,  linux-kernel@...r.kernel.org,
  linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH] PCI: endpoint: pci-epf-test: Limit PCIe BAR size for
 fixed BARs

On Thu 04 Sep 2025 at 23:29, Marek Vasut <marek.vasut@...lbox.org> wrote:

> On 9/4/25 2:28 PM, Niklas Cassel wrote:
>
> Hello Niklas,
>
> [...]
>
>> pci_epf_alloc_space() works like this:
>> If the user requests a BAR size that is smaller than the fixed-size BAR,
>> it will allocate space matching the fixed-size.
>> As in most cases, having a BAR larger than needed by an EPF driver is
>> still acceptable.
>> However, if the user requests a size larger than the fixed-size BAR,
>> as in your case, we will return an error, as we cannot fulfill the
>> user's request.
>> I don't see any alternative other than your/Damien's proposal above.
>> Unfortunately, all EPF drivers would probably need this same change.
>
> It seems that pci-epf-ntb and pci-epf-vntb only use BAR0 (BAR_CONFIG) and
> BAR0+BAR1 (BAR_CONFIG and BAR_DB) , so those should be OK on this
> controller. NVMe EPF also seems to use only BAR0 and it specifically
> handles fixed size BAR. It seems everything that is in the tree so far
> managed to sidestep hitting fixed-size BAR4 problems on this hardware,
> except for the test driver.

As it stands, a vNTB device needs 3 BARs minimum (CFG, DB and MW). The
NTB one may get away with with 2 BARs, with DB and MW sharing one.

If you referring to Renesas about that BAR4, I did use it for vNTB.
It is indeed not upstream ... yet [1]

I think it is possible to have vNTB on 2 BARs with some tweaks, putting
CFG and DB on the same one.

[1]: https://lore.kernel.org/r/20250702-ntb-rcar-support-v3-2-4268d9c85eb7@baylibre.com

-- 
Jerome