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

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Message-ID: <1956a94e-b231-4458-a1c1-6d9f158da669@quicinc.com>
Date: Wed, 19 Mar 2025 12:03:00 -0700
From: Wesley Cheng <quic_wcheng@...cinc.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Melody Olvera
	<quic_molvera@...cinc.com>,
        Vinod Koul <vkoul@...nel.org>,
        "Kishon Vijay
 Abraham I" <kishon@...nel.org>,
        Rob Herring <robh@...nel.org>,
        "Krzysztof
 Kozlowski" <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Philipp Zabel
	<p.zabel@...gutronix.de>,
        Bjorn Andersson <andersson@...nel.org>,
        "Konrad
 Dybcio" <konradybcio@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
CC: <linux-arm-msm@...r.kernel.org>, <linux-phy@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-usb@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 6/9] phy: qcom: Add M31 based eUSB2 PHY driver

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@...cinc.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@...cinc.com>
>> Signed-off-by: Melody Olvera <quic_molvera@...cinc.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