Re: [PATCH v3 2/2] PCI: imx6: limit DBI register length

From: Stefan Agner <stefan@agner.ch>
To: Trent Piepho <tpiepho@impinj.com>
Cc: l.stach@pengutronix.de, lorenzo.pieralisi@arm.com,
	leonard.crestez@nxp.com, jingoohan1@gmail.com,
	linux-kernel@vger.kernel.org, andrew.smirnov@gmail.com,
	hongxing.zhu@nxp.com, gustavo.pimentel@synopsys.com,
	linux-pci@vger.kernel.org, bhelgaas@google.com
Subject: Re: [PATCH v3 2/2] PCI: imx6: limit DBI register length
Date: Tue, 20 Nov 2018 21:42:28 +0100	[thread overview]
Message-ID: <268e109e1c6b309454bd5a313078894c@agner.ch> (raw)
In-Reply-To: <1542741198.30311.608.camel@impinj.com>

On 20.11.2018 20:13, Trent Piepho wrote:
> On Tue, 2018-11-20 at 18:19 +0000, Leonard Crestez wrote:
>> On Tue, 2018-11-20 at 17:56 +0100, Stefan Agner wrote:
>> > Define the length of the DBI registers. This makes sure that
>> > the kernel does not access registers beyond that point, avoiding
>> > the following abort on a i.MX 6Quad:
>> >   # cat
>> > /sys/devices/soc0/soc/1ffc000.pcie/pci0000\:00/0000\:00\:00.0/config
>> >   [  100.021433] Unhandled fault: imprecise external abort (0x1406)
>> > at 0xb6ea7000
>> >   ...
>> >   [  100.056423] PC is at dw_pcie_read+0x50/0x84
>> >   [  100.060790] LR is at dw_pcie_rd_own_conf+0x44/0x48
>>
>> I don't know exactly where this limitation comes from, I can indeed
>> reproduce a stack dump when dumping pci config from /sys/
>>
>> Unfortunately this seems to block access to registers used for
>> functionality like interrupts. For example dw_handle_msi_irq does:
>>
>> 	dw_pcie_rd_own_conf(pp, PCIE_MSI_INTR0_STATUS +
>> 			    (i * MSI_REG_CTRL_BLOCK_SIZE),
>> 			    4, &val);
>>
>> where PCI_MSI_INTR0_STATUS is 0x830. There are more accesses like this.
>>
>> Testing on 6dl-sabreauto (dts change required) with an ath9k pcie card
>> with your series I sometimes get "irq 295: nobody cared" on boot. Maybe
>> I'm missing something?
> 
> On IMX7d, there are significant blocks of 00s in the config space, and
> all 0xff at 0xb50 on up.
> 
> I.e., significant portions are empty, in the middle of the config
> space, not just at the end.
> 
> But they can be read without problem.
> 
> Perhaps imx6q aborts on a read of an unimplemented address instead of
> returning zeros like imx7d.  In that case it really needs something
> more complex to prevent abort than just a length.

Yeah it seems those SoCs behave differently.

Describing a register set with holes will get complicated, I guess it
would ask for a regmap...

> 
> It also seems to me that this doesn't need to be in the internal pci
> config access functions.  The driver shouldn't be reading registers
> that don't exist anyway.  It's really about trying to fix sysfs access
> to registers that don't exist.  So maybe it should be done there.

That was my first approach, see:
https://lkml.org/lkml/2018/11/14/716

--
Stefan