Re: [PATCH] pci: imx6: support kernels built in Thumb-2 mode

[Stefan Agner]

On 28.11.2018 17:16, Robin Murphy wrote:
> Hi Stefan,
> 
> On 28/11/2018 13:25, Stefan Agner wrote:
>> Add a fault handler which handles reads in Thumb-2 mode. Install
>> the appropriate handler depending on which mode the kernel has
>> been built. This avoids an "Unhandled fault: external abort on
>> non-linefetch (0x1008) at 0xf0a80000" during boot on a device
>> with a PCIe switch connected.
>>
>> Link: https://lore.kernel.org/linux-pci/20181126161645.8177-1-stefan@xxxxxxxx/
>> Signed-off-by: Stefan Agner <stefan@xxxxxxxx>
>> ---
>> FWIW, I found this manual helpful to write the code below:
>> http://hermes.wings.cs.wisc.edu/files/Thumb-2SupplementReferenceManual.pdf#page=43&zoom=100,0,66
> 
> This one's rather less ancient and even more authoritative ;)
> 
> https://static.docs.arm.com/ddi0406/cd/DDI0406C_d_armv7ar_arm.pdf
> 
> (ARMv7 had a few new encodings over and above ARMv6T2, although in
> fairness I don't think any should be relevant to this specific case)
> 

Thanks, I tried to find the right document on arm.com, but I timed out
after 5 minutes or so :-)

>> --
>> Stefan
>>
>>   drivers/pci/controller/dwc/pci-imx6.c | 37 ++++++++++++++++++++++++++-
>>   1 file changed, 36 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
>> index 69f86234f7c0..683deb74d69f 100644
>> --- a/drivers/pci/controller/dwc/pci-imx6.c
>> +++ b/drivers/pci/controller/dwc/pci-imx6.c
>> @@ -29,6 +29,7 @@
>>   #include <linux/reset.h>
>>   #include <linux/pm_domain.h>
>>   #include <linux/pm_runtime.h>
>> +#include <asm/opcodes.h>
>>     #include "pcie-designware.h"
>>   @@ -299,6 +300,37 @@ static int imx6q_pcie_abort_handler(unsigned long addr,
>>   	return 1;
>>   }
>>   +static int imx6q_pcie_abort_handler_thumb2(unsigned long addr,
>> +		unsigned int fsr, struct pt_regs *regs)
>> +{
>> +	unsigned long pc = instruction_pointer(regs);
>> +	unsigned long instr = *(unsigned long *)pc;
>> +	unsigned long thumb2_instr = __mem_to_opcode_thumb16(instr);
>> +	int reg = thumb2_instr & 7;
>> +
>> +	if (!__opcode_is_thumb16(instr & 0x0000ffffUL))
>> +		return 1;
> 
> There are plenty of 32-bit Thumb encodings of various LDR/STR
> variants, and I doubt we can guarantee that the offset, target
> register, and/or addressing mode for a config space access will
> *always* suit the (relatively limited) 16-bit ones.

Hm, I guess they should be handled too?

I looked at the code where I had the abort at hand triggered
(dw_pcie_read).
 
>> +
>> +	/* Load word/byte and halfword immediate offset */
>> +	if (((thumb2_instr & 0xe800) == 0x6800) ||
>> +	    ((thumb2_instr & 0xf800) == 0x8800)) {
>> +		unsigned long val;
>> +
>> +		if (thumb2_instr & 0x1000)
>> +			val = 0xff;
>> +		else if (thumb2_instr & 0x8000)
>> +			val = 0xffff;
>> +		else
>> +			val = 0xffffffffUL;
>> +
>> +		regs->uregs[reg] = val;
>> +		regs->ARM_pc += 2;
>> +		return 0;
>> +	}
> 
> What about stores? The existing implementation handles them, so either
> that's dead code which could perhaps be cleaned up, or they need to be
> handled here too.
> 

I think the current handler (imx6q_pcie_abort_handler) checks bit 20,
which means it must be a load not?

>> +
>> +	return 1;
>> +}
>> +
>>   static int imx6_pcie_attach_pd(struct device *dev)
>>   {
>>   	struct imx6_pcie *imx6_pcie = dev_get_drvdata(dev);
>> @@ -1069,6 +1101,8 @@ static struct platform_driver imx6_pcie_driver = {
>>     static int __init imx6_pcie_init(void)
>>   {
>> +	bool thumb2 = IS_ENABLED(CONFIG_THUMB2_KERNEL);
> 
> Can these aborts definitely *only* be triggered by kernel accesses,
> and never, say, via an mmap() of anything exposed to userspace?

Honestly, I am not very familiar with PCIe, I don't know...

--
Stefan

> 
> Robin.
> 
>> +
>>   	/*
>>   	 * Since probe() can be deferred we need to make sure that
>>   	 * hook_fault_code is not called after __init memory is freed
>> @@ -1076,7 +1110,8 @@ static int __init imx6_pcie_init(void)
>>   	 * we can install the handler here without risking it
>>   	 * accessing some uninitialized driver state.
>>   	 */
>> -	hook_fault_code(8, imx6q_pcie_abort_handler, SIGBUS, 0,
>> +	hook_fault_code(8, thumb2 ? imx6q_pcie_abort_handler_thumb2 :
>> +			imx6q_pcie_abort_handler, SIGBUS, 0,
>>   			"external abort on non-linefetch");
>>     	return platform_driver_register(&imx6_pcie_driver);
>>