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: 6fec018a7e70 ("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>

diff --git a/drivers/usb/gadget/function/u_audio.c b/drivers/usb/gadget/function/u_audio.c
index c46400b..4fb05f9 100644 (file)
--- 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;