wifi: ath12k: decrease MHI channel buffer length to 8KB

Currently buf_len field of ath12k_mhi_config_wcn7850 is assigned
with 0, making MHI use a default size, 64KB, to allocate channel
buffers. This is likely to fail in some scenarios where system
memory is highly fragmented and memory compaction or reclaim is
not allowed.

For now we haven't get any failure report on this in ath12k, but
there indeed is one such case in ath11k [1].

Actually those buffers are used only by QMI target -> host communication.
And for WCN7850, the largest packet size for that is less than 6KB. So
change buf_len field to 8KB, which results in order 1 allocation if page
size is 4KB. In this way, we can at least save some memory, and as well
as decrease the possibility of allocation failure in those scenarios.

[1] https://lore.kernel.org/ath11k/96481a45-3547-4d23-ad34-3a8f1d90c1cd@suse.cz/

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0-03427-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.15378.4
Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.0.1-00029-QCAHKSWPL_SILICONZ-1
Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240412060620.27519-6-quic_bqiang@quicinc.com

drivers/net/wireless/ath/ath12k/mhi.c

@@ -140,7 +140,7 @@ const struct mhi_controller_config ath12k_mhi_config_wcn7850 = {
 	.max_channels = 128,
 	.timeout_ms = 2000,
 	.use_bounce_buf = false,
-	.buf_len = 0,
+	.buf_len = 8192,
 	.num_channels = ARRAY_SIZE(ath12k_mhi_channels_wcn7850),
 	.ch_cfg = ath12k_mhi_channels_wcn7850,
 	.num_events = ARRAY_SIZE(ath12k_mhi_events_wcn7850),