Re: [PATCH v2 6/9] phy: qcom: Add M31 based eUSB2 PHY driver

[Wesley Cheng <quic_wcheng@quicinc.com>]

Hi Konrad,

On 3/11/2025 4:19 AM, Konrad Dybcio wrote:
> On 3/4/25 10:56 PM, Melody Olvera wrote:
>> From: Wesley Cheng <quic_wcheng@quicinc.com>
>>
>> SM8750 utilizes an eUSB2 PHY from M31.  Add the initialization
>> sequences to bring it out of reset and into an operational state.  This
>> differs to the M31 USB driver, in that the M31 eUSB2 driver will
>> require a connection to an eUSB2 repeater.  This PHY driver will handle
>> the initialization of the associated eUSB2 repeater when required.
>>
>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
>> Signed-off-by: Melody Olvera <quic_molvera@quicinc.com>
>> ---
> 
> [...]
> 
>> +static int msm_m31_eusb2_write_readback(void __iomem *base, u32 offset,
>> +					const u32 mask, u32 val)
>> +{
>> +	u32 write_val;
>> +	u32 tmp;
>> +
>> +	tmp = readl_relaxed(base + offset);
>> +	tmp &= ~mask;
>> +	write_val = tmp | val;
>> +
>> +	writel_relaxed(write_val, base + offset);
>> +
>> +	tmp = readl_relaxed(base + offset);
>> +	tmp &= mask;
>> +
>> +	if (tmp != val) {
>> +		pr_err("write: %x to offset: %x FAILED\n", val, offset);
>> +		return -EINVAL;
>> +	}
>> +
>> +	return 0;
> 
> Is there a reason we need to read back every write?
> 
> Does this have to do with some funny write buffering?
> 

Probably because its just a form of write synchronization, since we're
using the relaxed variants.  If desired I can switch to just using writel
and remove the readback.

Thanks
Wesley Cheng