Subject: Re: [PATCH] usb: gadget: u_audio: fix calculations for small
 bInterval
To:     John Keeping <john@metanate.com>, linux-usb@vger.kernel.org
Cc:     Felipe Balbi <balbi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
References: <20220104183243.718258-1-john@metanate.com>
From:   Pavel Hofman <pavel.hofman@ivitera.com>
Message-ID: <e3cae3a2-ef5c-2188-7fcf-b5b7dd9e4ba8@ivitera.com>
Date:   Wed, 5 Jan 2022 11:36:56 +0100
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Dne 04. 01. 22 v 19:32 John Keeping napsal(a):
> If bInterval is 1, then p_interval is 8000 and p_interval_mil is 8E9,
> which is too big for a 32-bit value.  While the storage is indeed
> 64-bit, this value is used as the divisor in do_div() which will
> truncate it into a uint32_t leading to incorrect calculated values.
> 
> Switch back to keeping the base value in struct snd_uac_chip which fits
> easily into an int, meaning that the division can be done in two steps
> with the divisor fitting safely into a uint32_t on both steps.
> 
> Fixes: 6fec018a7e70 ("usb: gadget: u_audio.c: Adding Playback Pitch ctl for sync playback")
> Signed-off-by: John Keeping <john@metanate.com>


Tested-by: Pavel Hofman <pavel.hofman@ivitera.com>

> ---
>   drivers/usb/gadget/function/u_audio.c | 24 +++++++++++++-----------
>   1 file changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/u_audio.c b/drivers/usb/gadget/function/u_audio.c
> index c46400be5464..4fb05f9576a6 100644
> --- a/drivers/usb/gadget/function/u_audio.c
> +++ b/drivers/usb/gadget/function/u_audio.c
> @@ -76,8 +76,8 @@ struct snd_uac_chip {
>   	struct snd_pcm *pcm;
>   
>   	/* pre-calculated values for playback iso completion */
> -	unsigned long long p_interval_mil;
>   	unsigned long long p_residue_mil;
> +	unsigned int p_interval;
>   	unsigned int p_framesize;
>   };
>   
> @@ -194,21 +194,24 @@ static void u_audio_iso_complete(struct usb_ep *ep, struct usb_request *req)
>   		 * If there is a residue from this division, add it to the
>   		 * residue accumulator.
>   		 */
> +		unsigned long long p_interval_mil = uac->p_interval * 1000000ULL;
> +
>   		pitched_rate_mil = (unsigned long long)
>   				params->p_srate * prm->pitch;
>   		div_result = pitched_rate_mil;
> -		do_div(div_result, uac->p_interval_mil);
> +		do_div(div_result, uac->p_interval);
> +		do_div(div_result, 1000000);
>   		frames = (unsigned int) div_result;
>   
>   		pr_debug("p_srate %d, pitch %d, interval_mil %llu, frames %d\n",
> -				params->p_srate, prm->pitch, uac->p_interval_mil, frames);
> +				params->p_srate, prm->pitch, p_interval_mil, frames);
>   
>   		p_pktsize = min_t(unsigned int,
>   					uac->p_framesize * frames,
>   					ep->maxpacket);
>   
>   		if (p_pktsize < ep->maxpacket) {
> -			residue_frames_mil = pitched_rate_mil - frames * uac->p_interval_mil;
> +			residue_frames_mil = pitched_rate_mil - frames * p_interval_mil;
>   			p_pktsize_residue_mil = uac->p_framesize * residue_frames_mil;
>   		} else
>   			p_pktsize_residue_mil = 0;
> @@ -222,11 +225,11 @@ static void u_audio_iso_complete(struct usb_ep *ep, struct usb_request *req)
>   		 * size and decrease the accumulator.
>   		 */
>   		div_result = uac->p_residue_mil;
> -		do_div(div_result, uac->p_interval_mil);
> +		do_div(div_result, uac->p_interval);
> +		do_div(div_result, 1000000);
>   		if ((unsigned int) div_result >= uac->p_framesize) {
>   			req->length += uac->p_framesize;
> -			uac->p_residue_mil -= uac->p_framesize *
> -					   uac->p_interval_mil;
> +			uac->p_residue_mil -= uac->p_framesize * p_interval_mil;
>   			pr_debug("increased req length to %d\n", req->length);
>   		}
>   		pr_debug("remains uac->p_residue_mil %llu\n", uac->p_residue_mil);
> @@ -591,7 +594,7 @@ int u_audio_start_playback(struct g_audio *audio_dev)
>   	unsigned int factor;
>   	const struct usb_endpoint_descriptor *ep_desc;
>   	int req_len, i;
> -	unsigned int p_interval, p_pktsize;
> +	unsigned int p_pktsize;
>   
>   	ep = audio_dev->in_ep;
>   	prm = &uac->p_prm;
> @@ -612,11 +615,10 @@ int u_audio_start_playback(struct g_audio *audio_dev)
>   	/* pre-compute some values for iso_complete() */
>   	uac->p_framesize = params->p_ssize *
>   			    num_channels(params->p_chmask);
> -	p_interval = factor / (1 << (ep_desc->bInterval - 1));
> -	uac->p_interval_mil = (unsigned long long) p_interval * 1000000;
> +	uac->p_interval = factor / (1 << (ep_desc->bInterval - 1));
>   	p_pktsize = min_t(unsigned int,
>   				uac->p_framesize *
> -					(params->p_srate / p_interval),
> +					(params->p_srate / uac->p_interval),
>   				ep->maxpacket);
>   
>   	req_len = p_pktsize;
>