Re: [PATCH 09/22] PCI: aardvark: Use SET_PCI_ERROR_RESPONSE() when device not found

From: "Pali Rohár" <pali@kernel.org>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: "Naveen Naidu" <naveennaidu479@gmail.com>,
	"Rob Herring" <robh@kernel.org>,
	"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Thomas Petazzoni" <thomas.petazzoni@bootlin.com>,
	bhelgaas@google.com,
	linux-kernel-mentees@lists.linuxfoundation.org,
	"moderated list:PCI DRIVER FOR AARDVARK (Marvell Armada 3700)"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 09/22] PCI: aardvark: Use SET_PCI_ERROR_RESPONSE() when device not found
Date: Wed, 13 Oct 2021 19:59:28 +0200	[thread overview]
Message-ID: <20211013175928.3sqnvrv7qc727uja@pali> (raw)
In-Reply-To: <20211013021310.GA1864069@bhelgaas>

On Tuesday 12 October 2021 21:13:10 Bjorn Helgaas wrote:
> On Tue, Oct 12, 2021 at 09:29:28PM +0530, Naveen Naidu wrote:
> > On 11/10, Pali Rohár wrote:
> > > On Monday 11 October 2021 23:55:35 Naveen Naidu wrote:
> > > > On 11/10, Pali Rohár wrote:
> > > > > On Monday 11 October 2021 23:26:33 Naveen Naidu wrote:
> > > > > > An MMIO read from a PCI device that doesn't exist or doesn't respond
> > > > > > causes a PCI error.  There's no real data to return to satisfy the
> > > > > > CPU read, so most hardware fabricates ~0 data.
> > > > > > 
> > > > > > Use SET_PCI_ERROR_RESPONSE() to set the error response, when a faulty
> > > > > > read occurs.
> > > > > > 
> > > > > > This helps unify PCI error response checking and make error check
> > > > > > consistent and easier to find.
> > > > > > 
> > > > > > Compile tested only.
> > > > > > 
> > > > > > Signed-off-by: Naveen Naidu <naveennaidu479@gmail.com>
> > > > > > ---
> > > > > >  drivers/pci/controller/pci-aardvark.c | 8 ++++----
> > > > > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> > > > > > index 596ebcfcc82d..dc2f820ef55f 100644
> > > > > > --- a/drivers/pci/controller/pci-aardvark.c
> > > > > > +++ b/drivers/pci/controller/pci-aardvark.c
> > > > > > @@ -894,7 +894,7 @@ static int advk_pcie_rd_conf(struct pci_bus *bus, u32 devfn,
> > > > > >  	int ret;
> > > > > >  
> > > > > >  	if (!advk_pcie_valid_device(pcie, bus, devfn)) {
> > > > > > -		*val = 0xffffffff;
> > > > > > +		SET_PCI_ERROR_RESPONSE(val);
> > > > > 
> > > > > Hello! Now I'm looking at this macro, and should not it depends on
> > > > > "size" argument? If doing 8-bit or 16-bit read operation then should not
> > > > > it rather sets only low 8 bits or low 16 bits to ones?
> 
> > > Function itself is declared as:
> > > 
> > > static int advk_pcie_rd_conf(struct pci_bus *bus, u32 devfn, int where, int size, u32 *val);
> > > 
> > > And in "size" argument is stored number of bytes, kind of read operation
> > > (read byte, read word, read dword). In *val is then stored read value.
> > > For byte operation, just low 8 bits in *val variable are set.
> > > 
> > > Because *val is u32 it means that typeof(*val) is always 4 independently
> > > of the "size" argument.
> > > 
> > > For example other project U-Boot has also pci-aardvark.c driver and
> > > U-Boot has for (probably same) purpose pci_get_ff() macro, see:
> > > https://source.denx.de/u-boot/u-boot/-/blob/v2021.10/drivers/pci/pci-aardvark.c#L367
> > > 
> > > I'm not saying if current approach to always sets 0xffffffff
> > > (independently of "size" argument) is correct or not as I do not know
> > > it too! I'm just giving example that this PCI code has very similar
> > > implementation of other project (U-Boot) which sets number of ones based
> > > on the size argument.
> 
> I don't think there's an issue here.  advk_pcie_rd_conf() can set the
> whole u32 to 0xffffffff regardless of the "size" value because
> pci_bus_read_config_byte() and pci_bus_read_config_word() extract out
> the part they need:
> 
>   res = bus->ops->read(bus, devfn, pos, len, &data);              \
>   *value = (type)data;                                            \
> 
> where "type" is u8 or u16 (this is hard to grep for; it's in the
> PCI_OP_READ() macro in drivers/pci/access.c).

Ok! No problem if this is something which is not going to be changed.

> > I am not sure too, if we would like to have something like pci_get_ff()
> > which sets the return mask based on the size.
> 
> I'd like to see how pci_get_ff() works because this is potentially a
> widespread, invasive change and it's always better to copy a good
> existing design than to make up something new.

Here is U-Boot implementation of that function, it is pretty simple:
https://source.denx.de/u-boot/u-boot/-/blob/v2021.10/drivers/pci/pci-uclass.c#L103-113

> > > Anyway, I very like this your idea to have a macro which purpose is to
> > > explicitly indicate error during config read operation! And to unify all
> > > drivers to use same style for signalling config read error.
> 
> I definitely think there's a lot of value in making this consistent
> *somehow*, so thanks for working on this!

+1