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SubjectRe: [PATCH v4 1/3] bus: mhi: core: Introduce internal register poll helper function
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On 3/11/2021 1:00 AM, Loic Poulain wrote:
> Hi Bhaumik,
>
> On Thu, 11 Mar 2021 at 00:31, Bhaumik Bhatt <bbhatt@codeaurora.org> wrote:
>>
>> Introduce helper function to allow MHI core driver to poll for
>> a value in a register field. This helps reach a common path to
>> read and poll register values along with a retry time interval.
>>
>> Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
>> ---
>> drivers/bus/mhi/core/internal.h | 3 +++
>> drivers/bus/mhi/core/main.c | 23 +++++++++++++++++++++++
>> 2 files changed, 26 insertions(+)
>>
>> diff --git a/drivers/bus/mhi/core/internal.h b/drivers/bus/mhi/core/internal.h
>> index 6f80ec3..005286b 100644
>> --- a/drivers/bus/mhi/core/internal.h
>> +++ b/drivers/bus/mhi/core/internal.h
>> @@ -643,6 +643,9 @@ int __must_check mhi_read_reg(struct mhi_controller *mhi_cntrl,
>> int __must_check mhi_read_reg_field(struct mhi_controller *mhi_cntrl,
>> void __iomem *base, u32 offset, u32 mask,
>> u32 shift, u32 *out);
>> +int __must_check mhi_poll_reg_field(struct mhi_controller *mhi_cntrl,
>> + void __iomem *base, u32 offset, u32 mask,
>> + u32 shift, u32 val, u32 delayus);
>> void mhi_write_reg(struct mhi_controller *mhi_cntrl, void __iomem *base,
>> u32 offset, u32 val);
>> void mhi_write_reg_field(struct mhi_controller *mhi_cntrl, void __iomem *base,
>> diff --git a/drivers/bus/mhi/core/main.c b/drivers/bus/mhi/core/main.c
>> index 4e0131b..7c7f41a 100644
>> --- a/drivers/bus/mhi/core/main.c
>> +++ b/drivers/bus/mhi/core/main.c
>> @@ -4,6 +4,7 @@
>> *
>> */
>>
>> +#include <linux/delay.h>
>> #include <linux/device.h>
>> #include <linux/dma-direction.h>
>> #include <linux/dma-mapping.h>
>> @@ -37,6 +38,28 @@ int __must_check mhi_read_reg_field(struct mhi_controller *mhi_cntrl,
>> return 0;
>> }
>>
>> +int __must_check mhi_poll_reg_field(struct mhi_controller *mhi_cntrl,
>> + void __iomem *base, u32 offset,
>> + u32 mask, u32 shift, u32 val, u32 delayus)
>> +{
>> + int ret;
>> + u32 out, retry = (mhi_cntrl->timeout_ms * 1000) / delayus;
>> +
>> + while (retry--) {
>> + ret = mhi_read_reg_field(mhi_cntrl, base, offset, mask, shift,
>> + &out);
>> + if (ret)
>> + return ret;
>> +
>> + if (out == val)
>> + return 0;
>> +
>> + udelay(delayus);
>
> Have you read my previous comment?
> Do you really want to risk hogging the CPU for several seconds? we
> know that some devices take several seconds to start/boot.
> Why not using msleep variant here?

usleep_range() if there is a desire to stay in us units?

Given that the use of this function is for 25ms in one case, I wonder if
this warning is applicable:
https://elixir.bootlin.com/linux/latest/source/include/linux/delay.h#L28

Counter point, 1ms latency over PCIe is not unusual. I know we've
removed the PCIe dependencies from MHI, but PCIe is the real usecase at
this time. Seems like this function could behave a bit weird if the
parameter to udelay is something like "100", but the
mhi_read_reg_field() call takes significantly longer than that. Feels
like in some scenarios, we could actually exceed the timeout by a
non-trivial margin.

I guess I'm going back and forth in determining if us scale timing is a
benefit in any way.

--
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

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