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

Arnd Bergmann
2020-01-23T11:39:26Z

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