Re: [PATCH v4 1/3] bus: mhi: core: Introduce internal register poll helper function

From: Bhaumik Bhatt
Date: Wed Mar 17 2021 - 17:27:50 EST

On 2021-03-11 11:59 AM, Jeffrey Hugo wrote:
On 3/11/2021 1:00 AM, Loic Poulain wrote:
Hi Bhaumik,

On Thu, 11 Mar 2021 at 00:31, Bhaumik Bhatt <bbhatt@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
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:

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.
Thanks for all the inputs. I think a good idea here would be to use fsleep()
API as we need to allow any timeout the caller specifies. Also, plan is to
drop the patch #3 in this series since that will require a busywait due to
the code being in panic path.

I don't wish to accommodate another variable here for busywait but that
would be an option to pick sleep or delay depending on the caller's path.

Please respond if there are any concerns.

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