usb: gadget: u_audio: fix calculations for small bInterval

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: 6fec018 ("usb: gadget: u_audio.c: Adding Playback Pitch ctl for sync playback")
Tested-by: Pavel Hofman <pavel.hofman@ivitera.com>
Signed-off-by: John Keeping <john@metanate.com>
Link: https://lore.kernel.org/r/20220104183243.718258-1-john@metanate.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: johnkeeping

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;