Re: [PATCHv10 5/6] lib: Add register read/write tracing support

[Arnd Bergmann]

On Thu, Feb 24, 2022 at 7:07 AM Sai Prakash Ranjan
<quic_saipraka@xxxxxxxxxxx> wrote:
>
> From: Prasad Sodagudi <psodagud@xxxxxxxxxxxxxx>
>
> Generic MMIO read/write i.e., __raw_{read,write}{b,l,w,q} accessors
> are typically used to read/write from/to memory mapped registers
> and can cause hangs or some undefined behaviour in following few
> cases,
>
> * If the access to the register space is unclocked, for example: if
>   there is an access to multimedia(MM) block registers without MM
>   clocks.
>
> * If the register space is protected and not set to be accessible from
>   non-secure world, for example: only EL3 (EL: Exception level) access
>   is allowed and any EL2/EL1 access is forbidden.
>
> * If xPU(memory/register protection units) is controlling access to
>   certain memory/register space for specific clients.
>
> and more...
>
> Such cases usually results in instant reboot/SErrors/NOC or interconnect
> hangs and tracing these register accesses can be very helpful to debug
> such issues during initial development stages and also in later stages.
>
> So use ftrace trace events to log such MMIO register accesses which
> provides rich feature set such as early enablement of trace events,
> filtering capability, dumping ftrace logs on console and many more.
>
> Sample output:
>
> rwmmio_write: __qcom_geni_serial_console_write+0x160/0x1e0 width=32 val=0xa0d5d addr=0xfffffbfffdbff700
> rwmmio_post_write: __qcom_geni_serial_console_write+0x160/0x1e0 width=32 val=0xa0d5d addr=0xfffffbfffdbff700
> rwmmio_read: qcom_geni_serial_poll_bit+0x94/0x138 width=32 addr=0xfffffbfffdbff610
> rwmmio_post_read: qcom_geni_serial_poll_bit+0x94/0x138 width=32 val=0x0 addr=0xfffffbfffdbff610
>
> Signed-off-by: Prasad Sodagudi <psodagud@xxxxxxxxxxxxxx>
> Co-developed-by: Sai Prakash Ranjan <quic_saipraka@xxxxxxxxxxx>
> Signed-off-by: Sai Prakash Ranjan <quic_saipraka@xxxxxxxxxxx>

I think this is ok in general. I saw that Steve had a minor comment, and
I suppose you could have just resent the same patches with a fixup in order
to have me pick it up into the asm-generic tree for 5.19.

There is one more thing that I saw looking through this patch again: the
address you print is the virtual __iomem token, but it might be more
valuable to have the physical address instead, which can be looked up
in the devicetree to know which register is affected.

There is a small extra cost to walk the page table, and I'm not sure
if we actually have an interface for it (vmalloc_to_page is almost
what we want, but it returns an invalid page pointer). Any suggestions
on this?

       Arnd