Re: [PATCH v14 1/2] drivers/coresight: Add UltraSoc System Memory Buffer driver

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 24/11/2022 13:33, hejunhao wrote:
> 
> On 2022/11/23 22:03, Suzuki K Poulose wrote:
>> On 23/11/2022 12:38, Junhao He wrote:
>>> From: Qi Liu <liuqi115@huawei.com>
>>>
>>> Add driver for UltraSoc SMB(System Memory Buffer) device.
>>> SMB provides a way to buffer messages from ETM, and store
>>> these "CPU instructions trace" in system memory.
>>> The SMB device is identifier as ACPI HID "HISI03A1". Device
>>> system memory address resources are allocated using the _CRS
>>> method and buffer modes is the circular buffer mode.
>>>
>>> SMB is developed by UltraSoc technology, which is acquired by
>>> Siemens, and we still use "UltraSoc" to name driver.
>>>
>>> Signed-off-by: Qi Liu <liuqi115@huawei.com>
>>> Signed-off-by: Junhao He <hejunhao3@huawei.com>
>>> Tested-by: JunHao He <hejunhao3@huawei.com>
>>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>>> ---
>>>   drivers/hwtracing/coresight/Kconfig        |  12 +
>>>   drivers/hwtracing/coresight/Makefile       |   1 +
>>>   drivers/hwtracing/coresight/ultrasoc-smb.c | 658 +++++++++++++++++++++
>>>   drivers/hwtracing/coresight/ultrasoc-smb.h | 129 ++++
>>>   4 files changed, 800 insertions(+)
>>>   create mode 100644 drivers/hwtracing/coresight/ultrasoc-smb.c
>>>   create mode 100644 drivers/hwtracing/coresight/ultrasoc-smb.h
>>>
>>
>>> +static void smb_sync_perf_buffer(struct smb_drv_data *drvdata,
>>> +                 struct cs_buffers *buf,
>>> +                 unsigned long head,
>>> +                 unsigned long data_size)
>>> +{
>>> +    struct smb_data_buffer *sdb = &drvdata->sdb;
>>> +    char **dst_pages = (char **)buf->data_pages;
>>> +    unsigned long to_copy;
>>> +    long pg_idx, pg_offset;
>>> +
>>> +    pg_idx = head >> PAGE_SHIFT;
>>> +    pg_offset = head & (PAGE_SIZE - 1);
>>> +
>>> +    while (data_size) {
>>> +        unsigned long pg_space = PAGE_SIZE - pg_offset;
>>> +
>>> +        /* Copy parts of trace data when read pointer wrap around */
>>> +        if (sdb->rd_offset + pg_space > sdb->buf_size)
>>> +            to_copy = sdb->buf_size - sdb->rd_offset;
>>> +        else
>>> +            to_copy = min(data_size, pg_space);
>>> +
>>> +        memcpy(dst_pages[pg_idx] + pg_offset,
>>> +                  sdb->buf_base + sdb->rd_offset, to_copy);
>>> +
>>> +        pg_offset += to_copy;
>>> +        if (pg_offset >= PAGE_SIZE) {
>>> +            pg_offset = 0;
>>> +            pg_idx++;
>>> +            pg_idx %= buf->nr_pages;
>>> +        }
>>> +        data_size -= to_copy;
>>> +        sdb->rd_offset += to_copy;
>>> +        sdb->rd_offset %= sdb->buf_size;
>>> +    }
>>> +
>>> +    sdb->data_size = 0;
>>
>>
>> --8>-- cut here --<8--
>>
>>> +    writel(sdb->start_addr + sdb->rd_offset,
>>> +        drvdata->base + SMB_LB_RD_ADDR_REG);
>>> +
>>> +    /*
>>> +     * Data remained in link cannot be purged when SMB is full, so
>>> +     * synchronize the read pointer to write pointer, to make sure
>>> +     * these remained data won't influence next trace.
>>> +     */
>>> +    if (sdb->full) {
>>> +        smb_purge_data(drvdata);
>>> +        writel(readl(drvdata->base + SMB_LB_WR_ADDR_REG),
>>> +               drvdata->base + SMB_LB_RD_ADDR_REG);
>>> +    }
>>
>> --<8-- end here --8>--
>>
>> As pointed out in the last review, we must do this step
>> everytime for perf mode irrespective of whether the buffer
>> was "FULL" or not.
>>
>> i.e, the above block should simply be:
>>
>>     if (sdb->full)
>>         smb_purge_data(drvdata);
>>
>>     /*
>>      * The uncollected Data must be discarded for perf,
>>      * as it cannot be clubbed with next schedule. We
>>      * any way TRUNCATE the buffer in this case.
>>      */
>>     writel(readl(drvdata->base + SMB_LB_WR_ADDR_REG),
>>         drvdata->base + SMB_LB_RD_ADDR_REG);
>>
>> Suzuki
> 
> Hi Suzuki,
> 
> We need to update SMB_LB_RD_ADDR_REG register first, then
> check the "full" flag, whether the register needs to be
> updated again.

Why ? sdb->full is not updated after the write to RD_ADDR_REG.

> 
> If we don`t update the value of SMB_LB_RD_ADDR_REG register
> or reset buffer state, the buffer state will still be "full".
> The buffer has not free area,so the data will still remain
> in link.

My issue here is with potentially "leaving the trace from a previous
session for the next session". i.e., at the end of a run, we must always
make sure that the buffer is left empty (unlike the sysfs mode).

e.g.,

perf_session_x: RUN0: RD=0x0, WR=0x5000, HANDLE_SIZE=0x3000, full=false.
At the end of the above routine we will have :
	RD=0x3000, WR=0x5000,

and if a different perf session comes in, say perf_session_y, it will 
consume trace written by "x" at 0x3000-0x50000, right ?

This is fine in the sysfs mode as we expect the entire sysfs mode
to be owned by a single session and is left to the user to split it.
But for perf mode we cannot do that and thus must make sure we don't
leak trace from one session to antoher.

Suzuki


> 
> Thanks.
> HeJunhao.
> 
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