linux-kernel - Re: [PATCH v3 1/2] PCI: Add enable_device() and disable

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Message-ID: <20241102124857.dx4hxdjy2jxjmara@thinkpad>
Date: Sat, 2 Nov 2024 18:18:57 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Frank Li <Frank.Li@....com>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Richard Zhu <hongxing.zhu@....com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	imx@...ts.linux.dev, alyssa@...enzweig.io, bpf@...r.kernel.org,
	broonie@...nel.org, jgg@...pe.ca, joro@...tes.org,
	lgirdwood@...il.com, p.zabel@...gutronix.de, robin.murphy@....com,
	will@...nel.org
Subject: Re: [PATCH v3 1/2] PCI: Add enable_device() and disable_device()
 callbacks for bridges

On Sat, Nov 02, 2024 at 12:24:42PM +0000, Marc Zyngier wrote:
> On Sat, 02 Nov 2024 11:54:35 +0000,
> Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org> wrote:
> > 
> > On Sat, Nov 02, 2024 at 11:32:37AM +0000, Marc Zyngier wrote:
> > > On Sat, 02 Nov 2024 11:10:12 +0000,
> > > Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org> wrote:
> > > > 
> > > > On Thu, Oct 24, 2024 at 06:34:44PM -0400, Frank Li wrote:
> > > > > Some PCIe host bridges require special handling when enabling or disabling
> > > > > PCIe Endpoints. For example, the i.MX95 platform has a lookup table to map
> > > > > Requester IDs to StreamIDs, which are used by the SMMU and MSI controller
> > > > > to identify the source of DMA accesses.
> > > > > 
> > > > > Without this mapping, DMA accesses may target unintended memory, which
> > > > > would corrupt memory or read the wrong data.
> > > > > 
> > > > > Add a host bridge .enable_device() hook the imx6 driver can use to
> > > > > configure the Requester ID to StreamID mapping. The hardware table isn't
> > > > > big enough to map all possible Requester IDs, so this hook may fail if no
> > > > > table space is available. In that case, return failure from
> > > > > pci_enable_device().
> > > > > 
> > > > > It might make more sense to make pci_set_master() decline to enable bus
> > > > > mastering and return failure, but it currently doesn't have a way to return
> > > > > failure.
> > > > > 
> > > > > Signed-off-by: Frank Li <Frank.Li@....com>
> > > > > ---
> > > > > Change from v2 to v3
> > > > > - use Bjorn suggest's commit message.
> > > > > - call disable_device() when error happen.
> > > > > 
> > > > > Change from v1 to v2
> > > > > - move enable(disable)device ops to pci_host_bridge
> > > > > ---
> > > > >  drivers/pci/pci.c   | 23 ++++++++++++++++++++++-
> > > > >  include/linux/pci.h |  2 ++
> > > > >  2 files changed, 24 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > > index 7d85c04fbba2a..5e0cb9b6f4d4f 100644
> > > > > --- a/drivers/pci/pci.c
> > > > > +++ b/drivers/pci/pci.c
> > > > > @@ -2056,6 +2056,7 @@ int __weak pcibios_enable_device(struct pci_dev *dev, int bars)
> > > > >  static int do_pci_enable_device(struct pci_dev *dev, int bars)
> > > > >  {
> > > > >  	int err;
> > > > > +	struct pci_host_bridge *host_bridge;
> > > > >  	struct pci_dev *bridge;
> > > > >  	u16 cmd;
> > > > >  	u8 pin;
> > > > > @@ -2068,9 +2069,16 @@ static int do_pci_enable_device(struct pci_dev *dev, int bars)
> > > > >  	if (bridge)
> > > > >  		pcie_aspm_powersave_config_link(bridge);
> > > > >  
> > > > > +	host_bridge = pci_find_host_bridge(dev->bus);
> > > > > +	if (host_bridge && host_bridge->enable_device) {
> > > > > +		err = host_bridge->enable_device(host_bridge, dev);
> > > > > +		if (err)
> > > > > +			return err;
> > > > > +	}
> > > > 
> > > > How about wrapping the enable/disable part in a helper?
> > > > 
> > > > 	int pci_host_bridge_enable_device(dev);
> > > > 	void pci_host_bridge_disable_device(dev);
> > > > 
> > > > The definition could be placed in drivers/pci/pci.h as an inline
> > > > function.
> > > 
> > > What does it bring? I would see the point if there was another user.
> > > But this is very much core infrastructure which doesn't lend itself to
> > > duplication.
> > > 
> > > Unless you have something in mind?
> > > 
> > 
> > IMO, it adds a nice encapsulation to help readers understand what this piece of
> > code is all about and also keeps the callers short. Plus the disable helper is
> > reused in both error and pci_disable_device() (if that matters).
> 
> Having an *internal* helper for disable definitely has its use.
> 
> But moving these helpers outside of pci.c opens the door to all sort
> of abuse by making it look like an internal API drivers can use, which
> is absolutely isn't.
> 

Hmm, agree with this part, thanks!

Frank, please keep the helpers in pci.c.

- Mani

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