linux-kernel - Re: [PATCH v2 1/4] usb: early: Avoid using DbC if already enabled

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Message-ID: <16400ee4-4406-8b26-10c0-a423b2b1fed0@gmail.com>
Date:   Mon, 17 May 2021 07:48:52 -0600
From:   Connor Davis <connojdavis@...il.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Jann Horn <jannh@...gle.com>, Lee Jones <lee.jones@...aro.org>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2 1/4] usb: early: Avoid using DbC if already enabled


On 5/17/21 3:32 AM, Jan Beulich wrote:
> On 14.05.2021 02:56, Connor Davis wrote:
>> Check if the debug capability is enabled in early_xdbc_parse_parameter,
>> and if it is, return with an error. This avoids collisions with whatever
>> enabled the DbC prior to linux starting.
> Doesn't this go too far and prevent use even if firmware (perhaps
> mistakenly) left it enabled?
>
> Jan

Yes, but how is one supposed to distinguish the broken firmware and 
non-broken

firmware cases?

>
>> Signed-off-by: Connor Davis <connojdavis@...il.com>
>> ---
>>   drivers/usb/early/xhci-dbc.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/drivers/usb/early/xhci-dbc.c b/drivers/usb/early/xhci-dbc.c
>> index be4ecbabdd58..ca67fddc2d36 100644
>> --- a/drivers/usb/early/xhci-dbc.c
>> +++ b/drivers/usb/early/xhci-dbc.c
>> @@ -642,6 +642,16 @@ int __init early_xdbc_parse_parameter(char *s)
>>   	}
>>   	xdbc.xdbc_reg = (struct xdbc_regs __iomem *)(xdbc.xhci_base + offset);
>>   
>> +	if (readl(&xdbc.xdbc_reg->control) & CTRL_DBC_ENABLE) {
>> +		pr_notice("xhci debug capability already in use\n");
>> +		early_iounmap(xdbc.xhci_base, xdbc.xhci_length);
>> +		xdbc.xdbc_reg = NULL;
>> +		xdbc.xhci_base = NULL;
>> +		xdbc.xhci_length = 0;
>> +
>> +		return -ENODEV;
>> +	}
>> +
>>   	return 0;
>>   }
>>   
>>
Thanks,

Connor