linux-kernel - Re: [PATCH] thunderbolt: Confirm the necessity to configure asym link first

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Message-ID: <877c0ylo2f.fsf@posteo.net>
Date: Thu, 26 Jun 2025 11:50:00 +0000
From: Charalampos Mitrodimas <charmitro@...teo.net>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc: zhangjianrong <zhangjianrong5@...wei.com>,  andreas.noever@...il.com,
  michael.jamet@...el.com,  YehezkelShB@...il.com,
  linux-usb@...r.kernel.org,  linux-kernel@...r.kernel.org,
  guhengsheng@...ilicon.com,  caiyadong@...wei.com,  xuetao09@...wei.com,
  lixinghang1@...wei.com
Subject: Re: [PATCH] thunderbolt: Confirm the necessity to configure asym
 link first

Mika Westerberg <mika.westerberg@...ux.intel.com> writes:

> Hi,
>
> On Thu, Jun 26, 2025 at 04:41:07PM +0800, zhangjianrong wrote:
>> Current implementation can cause allocation failures in
>> tb_alloc_dp_bandwidth() in some cases. For example:
>> allocated_down(30Gbps), allocated_up(50Gbps),
>> requested_down(10Gbps).
>
> I'm not sure I understand the above.
>
> Can you describe in which real life situation this can happen?

I suppose this can happen when reducing bandwidth while total upstream
bandwidth usage on the link exceeds TB_ASYM_MIN (36 Gbps). The
allocation fails at the asymmetric limit check before checking whether
the downstream request actually needs asymmetric mode.

>
>> 
>> Signed-off-by: zhangjianrong <zhangjianrong5@...wei.com>
>> ---
>>  drivers/thunderbolt/tb.c | 10 +++++-----
>>  1 file changed, 5 insertions(+), 5 deletions(-)
>> 
>> diff --git a/drivers/thunderbolt/tb.c b/drivers/thunderbolt/tb.c
>> index a7c6919fbf97..558455d9716b 100644
>> --- a/drivers/thunderbolt/tb.c
>> +++ b/drivers/thunderbolt/tb.c
>> @@ -1039,6 +1039,9 @@ static int tb_configure_asym(struct tb *tb, struct tb_port *src_port,
>>  			break;
>>  
>>  		if (downstream) {
>> +			/* Does consumed + requested exceed the threshold */
>> +			if (consumed_down + requested_down < asym_threshold)
>> +				continue;
>>  			/*
>>  			 * Downstream so make sure upstream is within the 36G
>>  			 * (40G - guard band 10%), and the requested is above
>> @@ -1048,20 +1051,17 @@ static int tb_configure_asym(struct tb *tb, struct tb_port *src_port,
>>  				ret = -ENOBUFS;
>>  				break;
>>  			}
>> -			/* Does consumed + requested exceed the threshold */
>> -			if (consumed_down + requested_down < asym_threshold)
>> -				continue;
>>  
>>  			width_up = TB_LINK_WIDTH_ASYM_RX;
>>  			width_down = TB_LINK_WIDTH_ASYM_TX;
>>  		} else {
>>  			/* Upstream, the opposite of above */
>> +			if (consumed_up + requested_up < asym_threshold)
>> +				continue;
>>  			if (consumed_down + requested_down >= TB_ASYM_MIN) {
>>  				ret = -ENOBUFS;
>>  				break;
>>  			}
>> -			if (consumed_up + requested_up < asym_threshold)
>> -				continue;
>>  
>>  			width_up = TB_LINK_WIDTH_ASYM_TX;
>>  			width_down = TB_LINK_WIDTH_ASYM_RX;
>> -- 
>> 2.34.1