Re: [PATCH 05/16] bus: mhi: core: Add support for ringing channel/event ring doorbells

[Arnd Bergmann]

On Thu, Jan 23, 2020 at 12:19 PM Manivannan Sadhasivam
<manivannan.sadhasivam@xxxxxxxxxx> wrote:

> +int __must_check mhi_read_reg(struct mhi_controller *mhi_cntrl,
> +                             void __iomem *base, u32 offset, u32 *out)
> +{
> +       u32 tmp = readl_relaxed(base + offset);
....
> +void mhi_write_reg(struct mhi_controller *mhi_cntrl, void __iomem *base,
> +                  u32 offset, u32 val)
> +{
> +       writel_relaxed(val, base + offset);

Please avoid using _relaxed accessors by default, and use the regular
ones instead. There are a number of things that can go wrong with
the relaxed version, so ideally each caller should have a comment
explaining why this instance is safe without the barriers and why it
matters to not have it.

If there are performance critical callers of mhi_read_reg/mhi_write_reg,
you could add mhi_read_reg_relaxed/mhi_write_reg_relaxed for those
and apply the same rules there.

Usually most mmio accesses are only needed for reconfiguration or
other slow paths.

      Arnd