linux-kernel - Re: [PATCH v8 0/5] PCI: endpoint: BAR subrange mapping support

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Message-ID: <ajuq5dpbzzoghd2hk7cz6w4t6jzcxfmygbyiciqjjhhg6ipkoh@usuhzmne6cap>
Date: Fri, 23 Jan 2026 09:36:26 +0900
From: Koichiro Den <den@...inux.co.jp>
To: Niklas Cassel <cassel@...nel.org>
Cc: Manivannan Sadhasivam <mani@...nel.org>, bhelgaas@...gle.com, 
	kwilczynski@...nel.org, frank.li@....com, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 0/5] PCI: endpoint: BAR subrange mapping support

On Thu, Jan 22, 2026 at 04:17:51PM +0100, Niklas Cassel wrote:
> On Thu, Jan 22, 2026 at 11:02:42PM +0900, Koichiro Den wrote:
> > On Thu, Jan 22, 2026 at 09:45:13AM +0100, Niklas Cassel wrote:
> > > Hello Koichiro,
> > > 
> > > On Thu, Jan 22, 2026 at 10:52:31AM +0900, Koichiro Den wrote:
> > > > > 
> > > > > In this series you didn't add a consumer of this subrange mapping, which I
> > > > > believe is the epf-vntb driver [1]. Even so I don't see a justification of why
> > > > > the BAR needs to be split into multiple subranges. Can you explain that?
> > > > 
> > > > Yes, the first consumer I have in mind is epf-vntb / remote eDMA-backed NTB
> > > > work referenced in [1].
> > > 
> > > Another consumer in your RFC series is pci-epf-test, specifically:
> > > [RFC PATCH v4 35/38] PCI: endpoint: pci-epf-test: Add pci_epf_test_next_free_bar() helper
> > > [RFC PATCH v4 36/38] PCI: endpoint: pci-epf-test: Add remote eDMA-backed mode
> > > [RFC PATCH v4 37/38] misc: pci_endpoint_test: Add remote eDMA transfer test mode
> > > [RFC PATCH v4 38/38] selftests: pci_endpoint: Add remote eDMA transfer coverage
> > > 
> > > where the pci-epf-test exports the eDMA registers (using subranges) over a
> > > BAR, and then in the host side driver, calls dw_edma_probe() on this subrange
> > > and uses the host side driver to control the eDMA residing in the endpoint.
> > > 
> > > Most of your patches in [RFC PATCH v4 35/38] have NTB prefix.
> > > 
> > > Would it not be possible to simply include patches 35-38 in this series,
> > > so we actually have a consumer, or do you need some other patches as well?
> > 
> > Sorry, I sent v9 just a few minutes after your email, without noticing it,
> > and then I was AFK for several hours.
> > 
> > > 
> > > E.g., perhaps
> > > [RFC PATCH v4 01/38] dmaengine: dw-edma: Export helper to get integrated register window
> > > 
> > > Would also be required?
> > 
> > Yes, please see the section "Patch layout" in:
> > https://lore.kernel.org/all/20260118135440.1958279-1-den@valinux.co.jp/
> > 
> >   [...]
> >   1. dw-edma / DesignWare EP helpers needed for remote embedded-DMA (export
> >      register/LL windows, IRQ routing control, etc.)
> > 
> >      Patch 01 : dmaengine: dw-edma: Export helper to get integrated register window
> >      Patch 02 : dmaengine: dw-edma: Add per-channel interrupt routing control
> >      Patch 03 : dmaengine: dw-edma: Poll completion when local IRQ handling is disabled
> >      Patch 04 : dmaengine: dw-edma: Add notify-only channels support
> >      Patch 05 : dmaengine: dw-edma: Add a helper to query linked-list region
> > 
> > Patches [RFC PATCH v4 01/38] through [RFC PATCH v4 05/38] are prerequisites
> > for the PCI test update, in addition to this "PCI: endpoint: BAR subrange
> > mapping support" series. Note that this series is described in "Kernel
> > base / dependencies" section of the large RFC series cover letter.
> > 
> > For this reason, those test cases cannot be included in this series.
> 
> Ok, I assumed that some of those 5 patches were either eDMA optimizations
> or things needed for NTB.
> 
> But if you need them even to just probe the eDMA driver on the host side
> (using registers from the remote eDMA), then perhaps it is too much.
> 
> I was hoping that we could at least have a very simple test,
> e.g. pci-endpoint-test only supporting READ or WRITE using remote eDMA.

If we were to add a simple test in this series, I think it would be
something like BAR_SUBRANGE_TEST (beside BAR_TEST), which dynamically
triggers subrange mapping on the EP side if supported, and performs
read/write to those subranges. That would keep the test focused on
validating the subrange mapping functionality, and would not involve remote
eDMA. What do you think?

Koichiro

> 
> At least then we would have a consumer part of this series.
> 
> pci-endpoint-test could have been further extended in a follow up series
> if there were some eDMA driver patches that were needed for more complex
> operations than a simple READ or WRITE using remote eDMA.
> 
> 
> Kind regards,
> Niklas