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nusb/rndis: avoid allocation on each transmission

The slack space for outgoing packets set to 44+16 bytes.

--- a/sys/src/cmd/nusb/ether/ether.c

+++ b/sys/src/cmd/nusb/ether/ether.c

@@ -290,12 +290,12 @@

 		n = 60;

 	/* slack space for header and trailers */

-	n += 2*16;

+	n += 44+16;

 	b = allocb(n);

 	/* header space */

-	b->wp += 16;

+	b->wp += 44;

 	b->rp = b->wp;

 	/* copy in the ethernet packet */

--- a/sys/src/cmd/nusb/ether/rndis.c

+++ b/sys/src/cmd/nusb/ether/rndis.c

@@ -95,19 +95,15 @@

 rndistransmit(Dev *ep, Block *b)

 {

 	int n;

-	uchar *req;

 	n = BLEN(b);

-	if((req = malloc(44 + n)) != nil){

-		PUT4(req, 1);      /* type = 1 (packet) */

-		PUT4(req+4, 44+n); /* len */

-		PUT4(req+8, 44-8); /* data offset */

-		PUT4(req+12, n);   /* data length */

-		memset(req+16, 0, 7*4);

-		memcpy(req+44, b->rp, n);

-		write(ep->dfd, req, 44+n);

-		free(req);

-	}

+	b->rp -= 44;

+	PUT4(b->rp, 1);      /* type = 1 (packet) */

+	PUT4(b->rp+4, 44+n); /* len */

+	PUT4(b->rp+8, 44-8); /* data offset */

+	PUT4(b->rp+12, n);   /* data length */

+	memset(b->rp+16, 0, 7*4);

+	write(ep->dfd, b->rp, 44+n);

 	freeb(b);

 }