linux-kernel - Re: [PATCH v8 3/6] PCI: endpoint: Add pci_epf_align

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Message-ID: <20241126041903.lq6zunvzoc2mmgbl@thinkpad>
Date: Tue, 26 Nov 2024 09:49:03 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Frank Li <Frank.li@....com>
Cc: Krzysztof Wilczyński <kw@...ux.com>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	imx@...ts.linux.dev, Niklas Cassel <cassel@...nel.org>,
	dlemoal@...nel.org, maz@...nel.org, tglx@...utronix.de,
	jdmason@...zu.us
Subject: Re: [PATCH v8 3/6] PCI: endpoint: Add pci_epf_align_addr() helper
 for address alignment

On Mon, Nov 25, 2024 at 02:22:23PM -0500, Frank Li wrote:
> On Sun, Nov 24, 2024 at 01:02:39PM +0530, Manivannan Sadhasivam wrote:
> > On Sat, Nov 16, 2024 at 09:40:43AM -0500, Frank Li wrote:
> > > Introduce the helper function pci_epf_align_addr() to adjust addresses
> >
> > pci_epf_align_inbound_addr()?
> >
> > > according to PCI BAR alignment requirements, converting addresses into base
> > > and offset values.
> > >
> > > Signed-off-by: Frank Li <Frank.Li@....com>
> > > ---
> > > change from v7 to v8
> > > - change name to pci_epf_align_inbound_addr()
> > > - update comment said only need for memory, which not allocated by
> > > pci_epf_alloc_space().
> > >
> > > change from v6 to v7
> > > - new patch
> > > ---
> > >  drivers/pci/endpoint/pci-epf-core.c | 45 +++++++++++++++++++++++++++++++++++++
> > >  include/linux/pci-epf.h             | 14 ++++++++++++
> > >  2 files changed, 59 insertions(+)
> > >
> > > diff --git a/drivers/pci/endpoint/pci-epf-core.c b/drivers/pci/endpoint/pci-epf-core.c
> > > index 8fa2797d4169a..4dfc218ebe20b 100644
> > > --- a/drivers/pci/endpoint/pci-epf-core.c
> > > +++ b/drivers/pci/endpoint/pci-epf-core.c
> > > @@ -464,6 +464,51 @@ struct pci_epf *pci_epf_create(const char *name)
> > >  }
> > >  EXPORT_SYMBOL_GPL(pci_epf_create);
> > >
> > > +/**
> > > + * pci_epf_align_inbound_addr() - Get base address and offset that match bar's
> >
> > BAR's
> >
> > > + *			  alignment requirement
> > > + * @epf: the EPF device
> > > + * @addr: the address of the memory
> > > + * @bar: the BAR number corresponding to map addr
> > > + * @base: return base address, which match BAR's alignment requirement, nothing
> > > + *	  return if NULL
> >
> > Below, you are updating 'base' only if it is not NULL. Why would anyone call
> > this API with 'base' and 'offset' set to NULL?
> 
> Some time, they may just want one of two.
> 

What would be the purpose? I fail to see it.

> >
> > > + * @off: return offset, nothing return if NULL
> > > + *
> > > + * Helper function to convert input 'addr' to base and offset, which match
> > > + * BAR's alignment requirement.
> > > + *
> > > + * The pci_epf_alloc_space() function already accounts for alignment. This is
> > > + * primarily intended for use with other memory regions not allocated by
> > > + * pci_epf_alloc_space(), such as peripheral register spaces or the trigger
> > > + * address for a platform MSI controller.
> > > + */
> > > +int pci_epf_align_inbound_addr(struct pci_epf *epf, enum pci_barno bar,
> > > +			       u64 addr, u64 *base, size_t *off)
> > > +{
> > > +	const struct pci_epc_features *epc_features;
> > > +	u64 align;
> > > +
> > > +	epc_features = pci_epc_get_features(epf->epc, epf->func_no, epf->vfunc_no);
> > > +	if (!epc_features) {
> > > +		dev_err(&epf->dev, "epc_features not implemented\n");
> > > +		return -EOPNOTSUPP;
> > > +	}
> > > +
> > > +	align = epc_features->align;
> > > +	align = align ? align : 128;
> > > +	if (epc_features->bar[bar].type == BAR_FIXED)
> > > +		align = max(epc_features->bar[bar].fixed_size, align);
> > > +
> > > +	if (base)
> > > +		*base = round_down(addr, align);
> > > +
> > > +	if (off)
> > > +		*off = addr & (align - 1);
> > > +
> > > +	return 0;
> > > +}
> > > +EXPORT_SYMBOL_GPL(pci_epf_align_inbound_addr);
> > > +
> > >  static void pci_epf_dev_release(struct device *dev)
> > >  {
> > >  	struct pci_epf *epf = to_pci_epf(dev);
> > > diff --git a/include/linux/pci-epf.h b/include/linux/pci-epf.h
> > > index 5374e6515ffa0..eff73ccb5e702 100644
> > > --- a/include/linux/pci-epf.h
> > > +++ b/include/linux/pci-epf.h
> > > @@ -238,6 +238,20 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
> > >  			  enum pci_epc_interface_type type);
> > >  void pci_epf_free_space(struct pci_epf *epf, void *addr, enum pci_barno bar,
> > >  			enum pci_epc_interface_type type);
> > > +
> > > +int pci_epf_align_inbound_addr(struct pci_epf *epf, enum pci_barno bar,
> > > +			       u64 addr, u64 *base, size_t *off);
> > > +static inline int pci_epf_align_inbound_addr_lo_hi(struct pci_epf *epf, enum pci_barno bar,
> > > +						   u32 low, u32 high, u64 *base, size_t *off)
> >
> > Why can't you just use pci_epf_align_inbound_addr() directly? Or the caller
> > could pass u64 address directly.
> 
> 
> msi message sperate low32 and high32.  (h << 32 | low) is quite easy to
> cause build warning.  it should be ((u64) h << 32) | low. Avoid copy this
> logic code at many EPF places.
> 

There is absolutely no overhead in doing so. Also the concern for me is,
pci_epf_align_inbound_addr() is exported but only used within
pci_epf_align_inbound_addr_lo_hi(). This causes confusion. So I'd prefer to have
a single exported API that is used by the callers.

- Mani

-- 
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