soc: qcom: smem: remove hwspinlock from item get routine

soc: qcom: smem: remove hwspinlock from item get routine

During an SSR(Sub-System Restart) process, the remoteproc driver will
try to read the crash reason from SMEM. The qcom_smem_get() backing such
operations does however take the hwspinlock (tcsr mutex), which might be
held by the dying remoteproc.

The associated timeout on the hwspin_lock_timeout_irqsave() would take
care of the system not hanging forever, but the get operation will fail,
unnecessarily delaying the process for the 'HWSPINLOCK_TIMEOUT' duration
(currently is '1s'), and finally resulting in failure to get crash
information from SMEM.

This timeout can be avoided by removing the hwspinlock in the
qcom_smem_get routine. SMEM ensures that the allocated item will only be
visible after the new item is safe to use by following a specific order
of updates.

In the private partition case, qcom_smem_get_private() will use
'offset_free_uncached' as a loop boundary when looking for existing
allocated items. The corresponding allocation will only update
offset_free_uncached once the item is fully initialized.

    hdr->canary = SMEM_PRIVATE_CANARY;
    hdr->item = cpu_to_le16(item);
    hdr->size = cpu_to_le32(ALIGN(size, 8));
    hdr->padding_data = cpu_to_le16(le32_to_cpu(hdr->size) - size);
    hdr->padding_hdr = 0;

    wmb();
    le32_add_cpu(&phdr->offset_free_uncached, alloc_size);

The global partition is similar but uses the "entry->allocated" variable
to ensure the item is not visible to qcom_smem_get_global().

Signed-off-by: Tao Zhang <quic_taozhan@quicinc.com>
Reviewed-by: Chris Lew <quic_clew@quicinc.com>
Link: https://lore.kernel.org/r/20240102022512.999635-1-quic_taozhan@quicinc.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>