[PATCHv5 4/4] asm-generic/io: Add logging support for MMIO accessors

[Arnd Bergmann]

On Mon, Dec 6, 2021 at 9:28 AM Sai Prakash Ranjan
<quic_saipraka at quicinc.com> wrote:
> +#if IS_ENABLED(CONFIG_TRACE_MMIO_ACCESS) && !(defined(__DISABLE_TRACE_MMIO__))
> +#include <linux/tracepoint-defs.h>
> +
> +DECLARE_TRACEPOINT(rwmmio_write);
> +DECLARE_TRACEPOINT(rwmmio_read);
> +
> +#define rwmmio_tracepoint_active(t) tracepoint_enabled(t)
> +void log_write_mmio(u64 val, u8 width, volatile void __iomem *addr);
> +void log_read_mmio(u8 width, const volatile void __iomem *addr);
> +
> +#else
> +
> +#define rwmmio_tracepoint_active(t) false
> +static inline void log_write_mmio(u64 val, u8 width, volatile void __iomem *addr) {}
> +static inline void log_read_mmio(u8 width, const volatile void __iomem *addr) {}
> +
> +#endif /* CONFIG_TRACE_MMIO_ACCESS */
>
>  /*
>   * __raw_{read,write}{b,w,l,q}() access memory in native endianness.
> @@ -149,6 +166,8 @@ static inline u8 readb(const volatile void __iomem *addr)
>  {
>         u8 val;
>
> +       if (rwmmio_tracepoint_active(rwmmio_read))
> +               log_read_mmio(8, addr);
>         __io_br();
>         val = __raw_readb(addr);
>         __io_ar(val);

For readability, it may be nicer to fold the two lines you add for each
helper into one, such as

void __log_write_mmio(u64 val, u8 width, volatile void __iomem *addr);
#define log_write_mmio(val, widtg, addr) do { \
     if (tracepoint_enabled(rwmmio_read)) \
               __log_write_mmio((val), (width), (addr)); \
} while (0)

I wonder if it may even be better to not check for tracepoint_active() in the
inline function at all but always enter the external function when built-in.
This means we do run into the branch, but it also reduces the i-cache footprint.

For general functionality, I think it would be better to trace the returned
value from the read, but I don't know if that defeats the purpose you
are interested in, since it requires the tracing to come after the __raw_read.

         Arnd