Re: [PATCH v2 2/2] PCI: rcar: Return all Fs from read which triggered an exception

[Arnd Bergmann <arnd@arndb.de>]

On Mon, Jan 17, 2022 at 11:03 PM <marek.vasut@gmail.com> wrote:
> It is possible to enforce the fault using 'isb' instruction placed
> right after the read/write instruction which started the faulting
> access. Add custom register accessors which perform the read/write
> followed immediately by 'isb'.
>
> This way, the fault always happens on the 'isb' and in case of read,
> which is located one instruction before the 'isb', it is now possible
> to fix up the return value of the read in the asynchronous external
> abort hook and make that read return all Fs.

Hi Marek,

As mentioned on IRC, I think this can be done a lot simpler, using a .text.fixup
section hack:
> +void rcar_pci_write_reg_workaround(struct rcar_pcie *pcie, u32 val, unsigned int reg)
> +{
> +#ifdef CONFIG_ARM
> +       asm volatile(
> +               "       str %0, [%1]\n"
> +               "       isb\n"
> +       ::"r"(val), "r"(pcie->base + reg):"memory");


I think this would looks something like

   int error = 0;
   asm volatile(
        "       str %1, [%2]\n"
        "1:       isb\n"
        "2:\n"
        "         pushsection .text.fixup,\"ax\"\n"
        "       .align  2\n"                                    \
        "3:     mov     %0, %3\n"                               \
        "       b       2b\n"                                   \
        "       .popsection\n"                                  \
        "       .pushsection __ex_table,\"a\"\n"                \
        "       .align  3\n"                                    \
        "       .long   1b, 3b\n"                               \
        "       .popsection"                                    \
       : "+r" (error) :"r"(val), "r"(pcie->base + reg), "i" (-ENXIO):"memory");

This saves you from hand-parsing the instruction sequence, which tends
to be even more fragile. After this, you just need to check the
'error' variable,
which remains at 0 normally but contains -ENXIO if an exception hits.

I'm not entirely sure this works for the particular exception you are getting,
and it probably requires not registering the rcar_pcie_aarch32_abort_handler
function, but it seems likely to work.

        Arnd